[advances in biochemical engineering/biotechnology] biomethanation ii volume 82 || applications of...

Applications of the Anaerobic Digestion Process

Irini Angelidaki 1 · Lars Ellegaard 2 · Birgitte Kiœr Ahring 3

1 Environment & Resources, The Technical University of Denmark, Block 115, 2800 Lyngby,Denmark. E-mail: [email protected]

2 BWSC, Gydevang 11, 3450 Allerød, Denmark3 Environmental Microbiology & Biotechnology, Biocentrum, The Technical University

of Denmark, Block 227, 2800 Lyngby, Denmark

At the start of the new millennium waste management has become a political priority in manycountries. One of the main problems today is to cope with an increasing amount of primarywaste in an environmentally acceptable way. Biowastes, i.e., municipal, agricultural or indus-trial organic waste, as well as contaminated soils etc., have traditionally been deposited in land-fills or even dumped into the sea or lakes without much environmental concern. In recenttimes, environmental standards of waste incineration and controlled land filling have gradu-ally improved, and new methods of waste sorting and resource/energy recovery have been de-veloped. Treatment of biowastes by anaerobic digestion processes is in many cases the optimalway to convert organic waste into useful products such as energy (in the form of biogas) anda fertilizer product. Other waste management options, such as land filling and incineration oforganic waste has become less desirable, and legislation, both in Europe and elsewhere, tendsto favor biological treatment as a way of recycling minerals and nutrients of organic wastesfrom society back to the food production and supply chain. Removing the relatively wet organicwaste from the general waste streams also results in a better calorific value of the remainderfor incineration, and a more stable fraction for land filling.

Keywords. Anaerobic digestion, Reactors, Codigestion, Biowastes, Solid waste, Slurries, Manure,Industrial waste

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Factors Influencing the Biogas Process . . . . . . . . . . . . . . . 4

2.1 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2 pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.3 Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Digestion of Slurries . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 Process and Plant Configuration . . . . . . . . . . . . . . . . . . 83.3 Veterinarian Aspects . . . . . . . . . . . . . . . . . . . . . . . . . 93.4 Combined Digestion of Livestock Waste and Industrial Waste . . 113.5 Process Tank Configuration . . . . . . . . . . . . . . . . . . . . . 133.6 Equipment Characteristics . . . . . . . . . . . . . . . . . . . . . 14

CHAPTER 1

Advances in Biochemical Engineering/Biotechnology, Vol. 82Series Editor: T. Scheper© Springer-Verlag Berlin Heidelberg 2003

3.6.1 Mixing Technique . . . . . . . . . . . . . . . . . . . . . . . . . . 143.6.2 Pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.6.3 Heat Exchanging . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.6.4 Biogas Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.6.5 Biogas Transmission . . . . . . . . . . . . . . . . . . . . . . . . . 203.6.6 Fiber Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.6.7 Odor Control Systems . . . . . . . . . . . . . . . . . . . . . . . . 223.7 Operational Experience and Results . . . . . . . . . . . . . . . . 223.7.1 Production Results . . . . . . . . . . . . . . . . . . . . . . . . . . 22

4 Digestion of High-Solid Wastes . . . . . . . . . . . . . . . . . . . 23

4.1 Pretreatment of Municipal Solid Waste . . . . . . . . . . . . . . . 244.2 Post Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.3 Biological Treatment . . . . . . . . . . . . . . . . . . . . . . . . . 254.3.1 Wet Digestion Systems . . . . . . . . . . . . . . . . . . . . . . . . 264.3.2 Dry digestion Systems . . . . . . . . . . . . . . . . . . . . . . . . 274.3.3 Multi-stage Anaerobic Digestion Systems . . . . . . . . . . . . . . 294.4 Summary of Processes Used for Anaerobic Treatment of Solid

Wastes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

1Introduction

Anaerobic degradation or digestion is a biological process where organic carbonis converted by subsequent oxidations and reductions to its most oxidized state(CO2), and its most reduced state (CH4).A wide range of microorganisms catalyzethe process in the absence of oxygen. The main products of the process are car-bon dioxide and methane, but minor quantities of nitrogen, hydrogen, ammoniaand hydrogen sulfide (usually less than 1% of the total gas volume) are also gen-erated. The mixture of gaseous products is termed biogas and the anaerobicdegradation process is often also termed the biogas process. As the result of theremoval of carbon, organic bound minerals and salts are released to their solu-ble inorganic form.

The biogas process is a natural process and occurs in a variety of anaerobic en-vironments. Such environments are, marine and fresh water sediment, sewagesludge, mud, etc.

The interest in the process is mainly due to the following two reasons:

– A high degree of reduction of organic matter is achieved with a small increase– in comparison to the aerobic process – in the bacterial biomass.

– The production of biogas, which can be utilized to generate different forms ofenergy (heat and electricity) or be processed for automotive fuel.

Biogas has a lower calorific value than natural gas, and in specific applications,such as automotive fuel, treatment of the biogas to improve its quality is required.

2 I. Angelidaki et al.

In Table 1, the upper and lower calorific value of biogas in comparison to naturalgas is shown.

The biogas process has been known and utilized for many years, but especiallyafter the rise of energy prices in the 1970s, the process received renewed atten-tion due to the need to find alternative energy sources to reduce the dependencyon fossil fuels.Although the price of fossil fuels decreased in 1985, the interest inthe biogas process still remains due to the environmental benefits of anaerobicwaste degradation. Additionally, the biomass used for biogas production wasoriginally produced by photosynthetic fixation of carbon dioxide from the at-mosphere, and combustion of biogas thus does not add extra carbon dioxide to the atmosphere as it does when combusting fossil fuels formed millions ofyears ago.

The anaerobic degradation process has been used for years for energy pro-duction and waste treatment. It is used in closed systems where optimal and con-trolled conditions can be maintained for the microorganisms. The process can beutilized for the fast and efficient degradation of different waste materials. Theanaerobic process is today mainly utilized in four sectors of waste treatment.

1. Treatment of primary and secondary sludge produced during aerobic treat-ment of municipal sewage. The process is utilized to stabilize and reduce thefinal amount of sludge and at the same time biogas is produced, which can beused to partly cover the need for energy in the sewage treatment plant. Thisapplication is widespread in the industrialized world in connection with theestablishment of advanced treatment systems for domestic wastewater.

2. Treatment of industrial wastewater produced from biomass-, food-processingor fermentation industries. These wastewater types are often highly loadedand can successfully be treated anaerobically before disposal directly to the en-vironment or sewage system. The produced biogas can often be utilized tocover the need for process energy. With the environmental concerns and costof alternative disposal this application of the anaerobic process is increasing.

3. Treatment of livestock waste in order to produce energy and improve the fer-tilizing qualities of manure. Due to more strict rules concerning the usage, dis-tribution, and storage of manure this application is growing especially incountries with a high animal production density.

4. A relatively new sector for use of the anaerobic processes on an industrial scaleis treatment of the organic fraction of municipal solid waste (OFMSW). Theaim of this process is first of all to reduce the amount of waste in other treat-ment systems, i.e. landfills and incineration plants, and secondly to recycle thenutrients from this type of waste to the agricultural sector.

Applications of the Anaerobic Digestion Process 3

Table 1. Calorific value of biogas and natural gas

Gas composition Biogas 65% CH4 Biogas 55% CH4 Natural gas

Upper calorific value KWh/m3STP a 7.1 6.0 12.0 Lower calorific value KWh/m3STP a 6.5 5.5 10.8

a STP (standard temperature and pressure), i.e. the volume at 0 °C and 1 bar pressure.

2Factors Influencing the Biogas Process

The biogas process as a complex biological process is influenced by several en-vironmental factors.

The interdependence of the bacteria is a key factor of the biogas process. Un-der conditions of unstable operation, intermediates such as volatile fatty acidsand alcohols accumulate at different rates depending on the substrate and thetype of perturbation causing instability. Thus, changes in the concentration of in-termediates indicate disturbance of the biogas process.

The most important environmental factors that can influence the balance ofthe system are temperature, pH, substrate composition and toxins.

2.1Temperature

Temperature is one of the main environmental factors affecting bacterial growth.Anaerobic bacteria are affected in the same way as the aerobic ones. Growth ratesoften increase with increasing temperature up to a certain limit, while there is arapid decrease in growth as the temperature approaches the upper limit for sur-vival of the bacterium. Besides the influence on growth rates of bacteria, tem-perature also influences physical parameters such as viscosity, surface tensionand mass transfer properties.Apart from the general steady state dependency ontemperature, also temperature stability is important, since even relatively smallchanges in the temperature result in an efficiency drop until adaptation has occurred.

Treatment of waste in anaerobic reactors is normally carried out within twotemperature ranges: around 25–40 °C, known as the mesophilic range, andhigher than 45 °C, known as the thermophilic range.

Several advantages of anaerobic waste digestion at thermophilic temperatureshave been reported previously.

At higher temperatures:

– the rate of digestion is faster, and thus shorter retention times are required,– smaller reactor volumes are required for treating the same amount of waste,– higher rate and efficiency of particulate matter hydrolysis,– more efficient destruction of pathogens.

Poor stability was previously believed to be associated with thermophilic tem-peratures. However, many years of experience with full-scale biogas plants op-erating at thermophilic temperature, have demonstrated that this is not the case.

Temperature has a positive effect on the digestion rate, resulting in higher volumetric methane production rates. Casali and Senior found that themethanogenic rate in refuse increased 2.6 times when the temperature was in-creased from ambient temperature to 30 °C and further 3 times when the tem-perature was increased from 30 to 40 °C.

However, the ultimate methane yield from organic matter is not influenced bytemperature in the range 30 to 60 °C. Consequently the effect of temperature on


growth rates can only be seen when the loading rates are high or retention timesshort. Varel et al. found with waste of beef-cattle in semi continuous reactor ex-periments that the effect of temperature on the rate of methane production wasmore noticeable at short retention times.As the retention time was decreased, ac-tive fermentation could only be achieved at thermophilic temperatures.At 3 daysRT, active fermentation could only be achieved at temperatures between 45 and60 °C and at 2.5 days RT only at 60 °C.

Methanogenesis is also possible under psychrophilic conditions (below 25 °C)but at lower process rates. Anaerobic bacteria can adapt quite easily to low tem-peratures, and high rate anaerobic treatment has been achieved at psychrophilicconditions, when bacteria have been immobilized or otherwise retained in theprocess.

It has been found that when shifting from mesophilic to psychrophilic tem-peratures the microbial populations still have the same composition as undermesophilic temperatures. This could indicate that at these temperatures psy-chrotolerant rather than true psychrophilic organisms are active.

2.2pH

The anaerobic digestion process is limited to a relatively narrow pH interval fromapprox. 6.0 to 8.5; a pH value outside this range can lead to imbalance.

Each of the microbial groups involved in anaerobic degradation has a specificpH optimum and can grow in a specific pH range. The methanogens and aceto-gens have pH optimum at approx. 7, while acidogens have lower pH optimumaround 6. Methanogens at pH lower than 6.6 grow very slowly.

In an anaerobic reactor, instability will as a rule lead to accumulation of VFA,which can lead to a drop in pH (acidification). However, accumulation of VFA willnot always be expressed as a pH drop due to the buffer capacity of some wastetypes. In manure there is a surplus of alkalinity, which means that the VFA ac-cumulation shall exceed a certain point before this can be detected as a signifi-cant change in pH. This means that when a drop in pH in the reactor is eventu-ally observed, the concentration of volatile fatty acids is most probably very highand the process may already have been affected.

There are many factors which influence pH. It is especially organic acids andcarbon dioxide which lower pH, while ammonia will contribute to an increase ofpH. Other compounds contributing to the buffering capacity are hydrogen sul-fide and phosphate.

2.3Toxicity

A number of compounds are toxic to the anaerobic microorganisms. Methan-ogens are commonly considered to be the most sensitive to toxicity of the micro-organisms in anaerobic digestion. However, the process can acclimatize, andhigher concentrations of the toxicant can be tolerated after a period of adapta-tion. The most common inhibitor for the anaerobic process is ammonia. In anaer-


obic digestion ammonia originates from soluble ammonia in the influent, fromprotein degradation and other compounds such as urea. Many substrates used foranaerobic treatment often contain ammonia in toxic concentrations. Such sub-strates include pig and poultry manure, slaughterhouse waste, potato, juice,highly proteinaceous sludge, wastewater from shale oil and coal liquefactionprocesses. The results concerning ammonia-N inhibitory level are conflicting, asthey depend on parameters such as pH, temperature and adaptation of the in-ocula. It is generally accepted that it is the non-ionized form of ammonia that isresponsible for inhibition, pH has a significant effect on the level of ammonia in-hibition, as the pH value determine the degree of ionization. The free ammoniaratio to total ammonia/ammonium ratio can be calculated from the equilibriumrelation as follows:

[NH3]/[T – NH3] = 1/(1+[H+]/Ka)

where, [NH3] and [T – NH3] are the free ammonia and the total ammonia/am-monium concentrations respectively, and Ka is the dissociation constant, whichis temperature dependent.

In Fig. 1 this dependency is shown graphically. As pH and temperature in-crease the free ammonia fraction also increases. Especially pH has a strong in-fluence on the degree of ionization of ammonia. Bhattacharya and Parkin foundthe maximum tolerable free-ammonia concentration to be only 55 mg-N/l. Mostreported inhibitory levels are, however, higher.Angelidaki and Ahring found that the biogas process could be adapted to tolerate free ammonia concentration of 800 mg-N/l. The inhibitory level of free ammonia depends strongly on the degree of acclimatization of inoculum to ammonia.

As the free ammonia concentration decreases with decreasing pH it could beexpected that even a slight reduction of pH would have a positive effect on ammo-


Fig. 1. pH and temperature influence on NH3 dissociation

nia inhibition. It was indeed found by several investigators that a pH decrease reduced ammonia inhibition. Free ammonia inhibition result in VFA accumula-tion, which in turn lower pH and decrease the ratio of free ammonia, with the result that free ammonia inhibition is relieved. Due to this self-stabilizing mech-anism, processes can be maintained in a stable ammonia inhibited state, where abalance between VFA concentration and ammonia loading exist.

The carbon/nitrogen (C/N) ratio is also important for process stability.A C/Nratio of 25 to 32 has been reported to have a positive effect on the methane yield.At lower C/N ratios the risk of excess nitrogen not needed for biomass synthesisand therefore becoming inhibitory increases. Opposite, a very high C/N ratiowould lead to N deficiency for biomass synthesis.Waste with very high COD con-centration and low content of nitrogen such as olive mill effluents has beenshown not to be able to be degraded alone. Addition of either nitrogen or codi-gestion with wastes with a lower C/N ratio was needed in order to digest olive milleffluents successfully.

Anaerobic treatment of wastewater containing high sulfate concentrations cancause inhibition as a result of the formation of hydrogen sulfide. It has been reported that total hydrogen sulfide concentrations of 100 to 300 mg/l or free hydrogen sulfide concentrations of 50 to 150 mg/l caused severe inhibition re-sulting in complete cessation of biogas production.

Long chain fatty acids, such as oleate and stearate, have been found to be toxicto the anaerobic process. No adaptation to the fatty-acid toxicity was observed.However, the presence of particulate material can increase the resistance of theprocess to long-chain fatty acids as LCFA are absorbed on the particulate mate-rial and thus not active as inhibitor. For other organic compounds such as phe-nols, chloroform and formaldehyde a reversible toxicity has been observed.Heavy metals are toxic for anaerobic microorganisms in concentrations in therange 10–3 to 10–4 M. However, experiments have shown that acclimatization tohigh heavy metal concentrations can occur and often the level of heavy metalswould become an environmental problem before affecting the process. In a re-actor, the actual concentration of soluble metal ions is normally low due to pre-cipitation of insoluble metal salts, e.g., as sulfides. It has been shown that less than2% of the metals may be in the soluble form.

Although anaerobic bacteria are inhibited by several toxicants they can alsotolerate a number of toxicants. Complete anaerobic degradation of pen-tachlorophenol has been reported. A number of other toxicants have been re-ported to be detoxified in anaerobic reactors such as nitroaromatic compounds,chlorinated aliphates, N-substituted aromatics and azo dyers.

3Digestion of Slurries

3.1General

One type of anaerobic waste treatment plants which have emerged since the mid1980s in Denmark and elsewhere, are large scale manure treatment plants serv-


ing a number of farmers, treating a slurry of manure, often with a fraction ofother waste as supplementary substrate.

The main objective is to extract energy and to improve the fertilizing qualityof manure, resulting in less nutrient leakage to ground and surface waters fromthe agricultural sector.

The description below gives an impression of the most important elements ofsuch a slurry treating biogas plant, also illustrating some of the practical designconsiderations not directly related to the reactor system itself, as well as illus-trating the gradual technical development waste treatment plants often undergo.

3.2Process and Plant Configuration

The main elements in a typical large-scale manure biogas plant, are shown inFig. 2. Raw material is transported by special constructed lorries from/to farm-ers and is kept in pre-storage buffer tanks. It is then pumped through heat ex-changers into the reactor at the desired flow rate. The effluent is pumped to an


Fig. 2. Concept of manure biogas plants top Farm biogas plant bottom Centralized biogas plant

after-storage tank, before it is brought back to the farmers and finally utilized onfields as fertilizer.

A biogas plant is in reality much more complex than described above. Manyunit operations are or may be utilized in such a plant i.e.:

– transport/pumping– stirring/mixing– macerating/grinding– heat exchanging– biogas treatment and cleaning– biogas compression and transportation– biogas storage– filtration/separation

Figure 3a shows a picture of a biogas plant (Blaabjerg biogas plant, Denmark),where some of these unit operations are employed, is shown.

In Fig. 4 the general lay-out of a full-scale biogas plant (Blaabjerg, Denmark)is shown.

Many process and lay-out variations are possible, often dictated by local con-ditions and needs.

3.3Veterinarian Aspects

Mixing and redistribution of manure, from several farms, requires adoption ofa sufficient level of sanitation, in order to avoid the risk of spreading pathogens.The veterinarian requirements in Denmark concerning manure are to ensurethat the manure is kept at a thermophilic temperature (> 50 °C) for a minimum


Fig. 3. A centralized biogas plant (Blaabjerg, Denmark)

of 4 hours. The required sanitation can be obtained directly in a thermophilicprocess by observing special pumping routines, whereas a mesophilic process re-quires a passive pre- or after sanitation stage (Fig. 5).

It has been documented that the above requirements ensure an effectivepathogen reduction, reaching a level equal to or better than 4 log-units when us-ing Faecal streptococcus as an indicator organism. Certain types of supplemen-tary waste suitable for codigestion in manure based biogas plants, such assewage-sludge, require more strict sanitation, due to the potential content of hu-man pathogens. Originally such types of waste required heating to 70 °C for aminimum of 1 hour.A special Danish veterinarian research effort, utilizing hightemperature resistant bacteria and vira as indicators, has resulted in the defini-tion of alternative temperature/time combinations, which in conjunction with abiogas process, are considered equivalent to a 70 °C/1 h heating. For a ther-mophilic biogas process (minimum 52 °C), a guaranteed temperature/retentiontime of 52 °C/10 hours, 53.5 °C/8 hours or 55 °C/6 hours is considered equivalentto 70 °C/1 hour. The retention can take place in separate sanitation tanks beforeor after the reactor stage, or directly in the reactor if pumping sequences allowthe necessary pauses. For mesophilic biogas plants slightly higher retention timesare required and sanitation must take place in a separate tank at a thermophilictemperature level.While inclusion of “problematic” waste in the past required aseparate pretreatment step, all the biomass can now be sufficiently sanitized ina single thermophilic process, provided the longer guaranteed retention times isbuilt into the design.


Fig. 4. Typical lay-out of a centralized biogas plant (Blaabjerg, Denmark). This plant is treat-ing approx. 100,000 ton of manure and industrial waste per year, and producing approx. 3 mil-lion m3 of biogas per year

3.4Combined Digestion of Livestock Waste and Industrial Waste

Construction of Joint Biogas Plants gives the possibility for combined anaerobictreatment and utilization of livestock waste and several types of organic wastefrom the food processing industry.

Organic industrial waste is usually characterized by high pollution loads andoften contains high concentrations of rapidly degradable substrates such as sac-charides, starches, lipids and proteins.

Manure usually has a rather low solids concentration (typically 3–5% totalsolids (TS) for pigs and 6–9% for cattle and dairy cows). In addition the manurecontain particles and straw/fibers with a content of ligno-cellulose. This fiberfraction is highly recalcitrant to degradation and will often pass through the re-actor mainly undigested. The high content of water together with the high frac-tion of fibers in manure is the reason for the low methane yields of manure, typ-ically ranging from 10–20 m3 CH4/ton of manure treated.

However, manure is an excellent basic substrate for co-digestion of industrialwaste, which could otherwise be difficult to process alone. The reasons for thisare:

– the high content of water in manure acts as a solvent for the dryer types ofwastes, solving problems of pumping and mechanical treatment of solidwastes.


Fig. 5. Plant configuration examples: 1 Thermophilic plant with two reactors and heat ex-changing; 2 Mesophilic plant with two reactors, heat exchanging and thermophilic post sani-tation

– the high buffering capacity of manure protects the process against failure dueto pH drop if the VFA concentration increase.

– manure is rich in a wide variety of nutrients necessary for optimal bacterialgrowth.

By combining different types of waste, such as manure, municipal solid waste and organic industrial waste, a much higher gas yield can be obtained from biogas reactors since organic industrial wastes in particular are more easily degraded and have a higher gas potential than manure. As most types of indus-trial organic waste result in methane yields varying from 30 to 500 m3/ton, thesesources constitute a very attractive supplementary substrate for manure basedbiogas plants.

The gas yields from different types of organic industrial waste is shown inTable 2.

Besides increasing the yield, addition of easily degradable material has beenshown to stabilize the anaerobic digestion process if added in a controlled fash-ion. This effect could be partly due to a higher active biomass concentration inthe reactor, which will be more resistant to inhibitory compounds. Furthermore,the inorganic parts of some organic wastes, such as clays and iron compounds,


Table 2. Methane yield from different types of industrial waste

Type of organic waste Composition of the Organic content Methane yieldorganic material (%) (m3/ton)

Stomach and Carbohydrates, proteins and lipids 15–20 40–60 intestine content

Flotation sludge 65–70% proteins, 30–35% lipids 13–18 80–130 (dewatered)

Bentonite- 70–75% lipids, 40–45 350–450bound oil 25–30% other organic matter

Fish-oil sludge 30–50% lipids and other 80–85 450–600organic matter

Source sorted organic Carbohydrates, proteins, and lipids 20–30 150–240household waste

Whey 75–80% lactose and 20–25% protein 7–10 40–55

Concentrated whey 75–80% lactose and 20–25% protein 18–22 100–130

Size water 70% proteins and 30% lipids 10–15 70–100

Marmelade 90% sugar, fruit organic acids 50 300

Soya oil/Margarine 90% vegetable oil 90 800–1000

Methylated spirits 40% alcohol 40 240

Sewage sludge Carbohydrates, lipids, proteins 3–4 17–22

Concentrated sewage Carbohydrates, lipids, proteins 15–20 85–110sludge

have been shown to counteract the inhibitory effect of ammonia and sulfide, re-spectively.

3.5Process Tank Configuration

Continuous feeding/pumping is often preferred in order to optimize heat exchangers. In order to obtain the necessary guaranteed retention time for sanitation purposes, configurations with two or three reactors (or sanitationtanks) operated in parallel are often seen, Fig. 6. At any given time, one reactoris being fed, one emptied and one resting, i.e., ensuring the sanitation retentiontime.

As fewer larger tanks are often preferred for economical reasons, variousschemes with discontinuous pumping or combinations with smaller buffer tankscan also be seen.


Fig. 6. A biogas plant with two reactors and post sanitation

3.6Equipment Characteristics

3.6.1Mixing Technique

Initially, submerged electrically driven medium speed mixers, mounted on guid-ing rails were utilized for most mixing tasks. These mixers soon proved rathercostly to operate, especially where access and routine service inspection is diffi-cult as is the case for digesters or other closed process tanks. Slow moving, topmounted central mixers with a freely suspended shaft with two propellers havebecome the preferred solution for reactor mixing. The mixers usually work con-tinuously with a mixing power input of 3–4 W/m3. Service requirements are verylimited, but a correct reactor level relative to the upper impeller is very criticalin order to avoid annoying floating layer formation. In storage tanks several typesof mixers are in use, including improved submerged models. The mixing energyinput is often discontinuous (high power for a short period) and average mixingpower input typically varies from 10 W/m3 in prestorage/mixing tanks to 1 W/m3

in after storage tanks. The position and type of mixers in combination with tankgeometry have proven to be very critical. Hydrodynamic favorable solutions, al-lowing the material to flow to the mixers, generally work best in combinationwith mixing at different depths. Figure 7 schematically shows the most commonmixer configurations.


Fig. 7. Common mixer configurations: A Top mounted reactor mixer; B Side mounted mixerin process tank/storage; C Submerged mixer on guiding rail; D Top mounted mixer; E Topmounted mixer with submerged angle gear

In Fig. 8 a picture of the reactor top with a centrally mounted top mixer isshown.

3.6.2Pumping

Eccentric worm pumps have been used for continuous feeding/extraction ofbiomass (low flow, high pressure) while centrifugal pumps with open impellersand cutters have been used for biomass transfer between storage tanks (highflow, low pressure). Feeding pumps, in particular those handling fresh biomasshave proven to be rather costly to operate due to high wear rates, but, despite several efforts, no viable alternative has been found so far. Low pump speed(< 300 rpm) and a pressure drop as low as possible can somewhat alleviate the problems.


Fig. 8. Reactor top, showing centrally mounted top mixer. Mixer shaft extends to the bottom ofthe reactor (approx. 17 m) with two propellers (3.2 m) placed at the top (just below liquid sur-face) and at the bottom

3.6.3Heat Exchanging

Most biogas plants in Denmark utilize heat exchanging between in-going freshbiomass and outgoing digested biomass, although a few plants have been builtwith no or limited heat exchanging. The choice depends on the value of heat, i.e.,the possibility of utilizing heat for other purposes.

Due to the high viscosity of manure and other biomass, especially when fresh,it is seldom possible to obtain a turbulent flow without generating an excessivepressure drop. Laminar or transition flow is the result with rather poor heattransfer coefficients. The most common types of exchangers utilize curved flowchannels to break up the thermal boundary layers by secondary flow or multipass design to break up the flow at regular intervals. Most designs attain heattransmission values of approx. 300 W/m2/°C with flowing velocities up to approx.1 m/s, when exchanging fresh biomass against digested biomass, or approx. twotimes as much if exchanging against water. Dimensioning, especially for pressuredrop, relies very much on experience, as prediction of biomass viscosity is veryuncertain.A good design margin and spare pump capacity is necessary to avoidproblems. In designs with rather narrow flow passages (< approx. 30 mm) it isconsidered wise to macerate the fresh biomass, which is also likely to improve thebiogas yield slightly.

Different heat exchanger configurations are shown in Fig. 9.Typical dimensioning practice (in Denmark) leads to heat exchanger instal-

lations with a resulting temperature difference of 10–15 °C. The remaining heat-


Fig. 9. Different heat exchanger configurations

ing up to process temperature, and for heat loss compensation, is by hot watercoils in the reactors or with a final biomass/water heat exchanger plus fewer coilsfor heat loss compensation. In this connection, it is important to recognize andinclude the heat loss due to evaporation of the water contained in the humid bio-gas leaving the reactor. A typical heat exchanger used in full scale biogas plantsis shown in Fig. 10.

On the input side of heat exchangers (and pumping systems) the risk of clog-ging is manly due to the possible presence of foreign objects. On the digestateside of heat exchangers, one must reckon with the formation of struvite(MgNH4PO4), which forms due to over-saturation when the biomass is cooled.The scaling occurs mainly in the cold sections and will gradually close flow chan-nels. The struvite needs to be dissolved by flushing with a circulating weak acidat regular intervals. Residual scaling also occurs in cold pipes down stream ofheat exchangers, although at a much slower rate.

3.6.4Biogas Cleaning

Dry biogas from manure generally consist of approx. 60–70% v/v methane(CH4), 30–40% v/v carbon dioxide (CO2), 1–2% v/v nitrogen (N2), 1000–3000 ppm hydrogen-sulfide (H2S) and 10–30 ppm NH3 . The concentration ofhydrogen-sulfide stated is typical when operating on manure alone. Other


Fig. 10. Typical exchanger used in full scale biogas plants

substrates, even in relatively small amounts, can result in quite different levels,both lower and higher.

Raw biogas is a humid gas, which needs to be cooled and carefully drained be-fore utilization. Early attempts to dry biogas using regenerative absorptionequipment designed for pressurized air proved difficult due to accelerated ab-sorbent decomposition, and the preferred solution has instead become to sim-ply cool the biogas and to design the gas system with care to avoid undrained lowpoints. In the case where the biogas is to be utilized in a boiler and emission re-quirements for sulfur-dioxide are not too strict, no further cleaning is necessary.In many cases, however, the biogas is utilized in gas engines for Combined Heatand Power (CHP) generation. In this case the hydrogen-sulfide content needs tobe kept below approx. 700 ppm for conventional gas engines in order to avoid ex-cessive corrosion and too rapid (and costly) lubrication oil deterioration. Onemethod of reducing the hydrogen-sulfide content is to add a commercial ferroussolution to the reactor feed. Ferrous compounds bind sulfur as insoluble prod-ucts in the liquid phase, which reduces the evolution of gaseous hydrogen-sulfide.The method has been utilized in a number of plants, but is rather costly if appliedcontinuously, since the consumption of ferrous material on a stoichiometric ba-sis has proven to be 2–3 times the desired reduction in gaseous hydrogen-sulfide.In some cases, however, ferrous containing waste products can be obtained forcodigestion most of the year, with commercial ferrous addition acting as a backup possibility only.

Often secondary H2S biogas cleaning is necessary, and in this case a biologi-cal oxidation process has become the dominating solution. By injection of a smallamount of air (2–8 % v/v) into the raw biogas, the hydrogen-sulfide content canbe biologically oxidized either to free (solid) sulfur or (aqueous) sulfurous acidaccording to following reactions:

2 H2S + O2 Æ H2O + 2 S (1)

2 H2S + 3 O2 Æ 2 H2SO3 (2)

The reaction will occur spontaneously and can take place in the reactor headspace on the floating layer (if any) and reactor walls, if air is injected directly in the headspace. Due to the acidic nature of the products with a risk of corrosion and the dependency of a stable floating layer, it is often preferred to isolate the process in a separate reactor as shown schematically in Fig. 11.

The reactor is somewhat similar to a scrubber, consisting of a porous filling(randomly packed plastic elements or similar) where microorganisms can grow,and a sump, pump, and nozzle arrangement allowing regular showering of thefilling. Showering has the function of washing out acidic products and supplyingnutrients to the microorganisms. The sump must therefore contain a liquid witha high alkalinity and contain essential nutrients, for which digested manure,preferably screened, is the ideal and readily available choice.A reactor loading ofapprox. 10 m3/h of biogas per m3 of reactor filling and a process temperaturearound 35 °C has been the normal choice, and the process has proven very effi-cient, provided that sufficient air is injected (slightly more than stoichiometri-


cally needed). Sump pH must be maintained at 6 or higher.A washing procedure,where the filling elements are gurgled through with an air/water mixture, has tobe carried out at regular intervals in order to prevent free sulfur deposits fromclosing the reactor filling.

In some cases, where biogas is stored or passing through a digestate after storage, the H2S reactor is omitted and only air injected. Cleaning is then relying on the formation of a floating layer in the after storage, on which the microorganisms can grow and perform the oxidation. A floating layer canusually be maintained with the choice of a low mixing intensity, without toomany problems for operating the tank as a buffer storage. This solution is certainly more cost effective, but can also be more unreliable as floating layers can be rather unstable, i.e., sinking overnight perhaps to resurface somedays later. At least some periods with reduced cleaning efficiency can be ex-pected.

In Fig. 12, a reactor used for biological hydrogen-sulfide reduction is shown.


Fig. 11. Schematic diagram of system for biological H2S oxidation

3.6.5Biogas Transmission

Biogas Plants are usually situated at some distance from the nearest town, whilebiogas utilization in an engine CHP installation often takes place inside or in thevicinity of the town in order to connect to a district heating system. As a conse-quence biogas often has to be transported up to several km between the plants.The first plants utilized high pressure (2–4 bar) dry transmission with pistoncompressors and absorption drying equipment.As mentioned earlier, absorptiondrying presented problems, and as piston compressors also proved vulnerableand operationally expensive, which has caused the low pressure wet transmissionto be gradually taken over. This solution usually involves a transmission pressurein the range 400–700 mbar in order to deliver at 100–200 mbar, utilizing Rootsblowers and a carefully planned buried transmission line, laid with a minimum3–5‰ slope (preferably in the flow direction) with a limited number of con-densate wells. The bulk of condensation occurs in the first few wells within100–500 m from the plant. Wells are equipped with valves for regular manualemptying, and the wells are usually designed to hold up to a weeks amount ofcondensate, meaning that the first wells must be quite large (typically 2 m3), whilethe last only need to be small (typically 100 liters).


Fig. 12. Hydrogen sulfide reduction reactor, 80 m3 with 50 m3 filling material. H2S is oxidizedby a biological process to acidic products or free sulfur, with injection of a small amount ofatmospheric air upstream of the reactor

At transmission distances up to a few hundred meters, high speed centrifugalblowers and a transmission pressure of approx. 100 mbar can be employed withadvantage.

3.6.6Fiber Separation

Manure and other types of biomass contain fibers (from straw) and other organicstructures which are difficult for anaerobic organisms to access.A typical degra-dation “efficiency” of 50–60% is often seen in manure based systems, meaningthat the digestate still contain 40–50% of the original organic dry matter contentprimarily as fibers. Some of the early plants included separators to take out a partof the fibers for production of commercial compost. Marketing this compost inrelatively small scale proved quite difficult, and compost production has ceasedin most plants. Some of the newest plants in Denmark again include separationequipment. The intention now is to produce a fiber fraction with a dry mattercontent in excess of 45%, which can be utilized as a supplementary fuel in woodchip boilers. This way the overall energy efficiency can be raised by approx. 15%through additional heat production. A side benefit, which in the near futuremight add to the feasibility of this scheme, is the removal of surplus phosphorouswhich is predominantly attached to the fibers.

Screw type separators dominate, sometimes in combination with a pre-sepa-ration in order to increase the capacity (Fig. 13).


Fig. 13. Fiber collection wagon, with distribution screw

3.6.7Odor Control Systems

One of the benefits from anaerobic digestion of manure is a considerable reduc-tion in odor nuisance when spreading manure on fields. The biogas plant itselfcan, however, give rise to local odor nuisance if care is not taken to limit odoremissions.

As a first preventive measure, it is of course desirable to situate the plant (ifpossible) without immediate neighbors. In addition all storage tanks should becovered. Due to service openings and as unloading/loading and internal pump-ing generates tank breathing, it is necessary to arrange a weak suction from thestorage tanks and other contaminated sections of the plant. This air must betreated in order to avoid odor emission.

Air treatment is usually chosen either as a biological cleaning in a compost filter or as a combustion. Compost filters are relatively cheap but operational experience. Careful attention and maintenance are required in order to ensure satisfactory cleaning, as filters sometimes fail due to acidification (oxidation ofhydrogen-sulfide), might collapse and become too dense, or might run dry. Com-bustion in a boiler in conjunction with generating process heat for the biogas planthas been tried, but the need for combustion air is limited, enforcing a compromiseon venting flow. Combustion in engines (together with the biogas) has so far notbeen attempted, due to the fear of affecting the engine negatively. In a few plants,a system involving combustion with regenerative heat exchanging has been em-ployed, a technique commonly utilized for the combustion of gaseous solvents inindustrial ventilation air.Although a rather expensive solution, results are positive.

3.7Operational Experience and Results

3.7.1Production Results

Figure 14 shows the average monthly biogas production during the first opera-tional period of two thermophilic Biogas plants, both with a biomass treatmentcapacity of 100,000 ton biomass per year. The figure shows typical start-up periods of 4–6 months and illustrates the relative stability of the production,once normal operation is obtained. The sudden rise in biogas production of oneof the biogas plants after month 13, was due to the introduction of residual fishoil as a co-substrate.

Consumption of process energy of relatively new plants typically amount toapprox. 4–5 kWh per m3 biomass treated for mixing, pumping, control etc., anda heat consumption of 15–25 kWh per m3 of biomass treated, including con-sumption for hot water, heating of buildings, etc. This should be compared to abiogas yield of approx. 30 m3 per m3 biomass, corresponding to a lower calorificvalue (LCV) yield of approx. 200 kWh, i.e., 10–15% of the energy produced isconsumed in the process. These figures represent the state of the art level forlarge-scale manure based biogas plants in Denmark, achieved as a result of thetechnical development described shortly above.


4Digestion of High-Solid Wastes

A special problem arises when solid wastes are to be treated.Anaerobic treatmentof waste water and other liquid types of wastes have been used for decades. How-ever, treatment of high-solids wastes such as municipal solid waste (MSW) is arelatively new application of anaerobic treatment, and many technical aspectsneed special consideration when inhomogeneous urban waste is the source ma-terial.

“High-solid wastes” might be defined as organic material with a content ofsolids between 10 to 40%, which is not fluid. The most important type of highsolid wastes is Municipal Solid Waste (MSW) or Household Solid Waste (HSW).

The most common way to dispose of MSW has until recently been land fillingor incineration. However, in the recent years attention has focused on recycling.By recycling the valuable elements of waste are converted to useful products andreturned to the supply chain, minimizing the pressure on land filling and incin-eration facilities, while also conserving primary resources.

Recycling started as a positive but minor part of waste management, but hasbecome a more and more dominant element in the waste policies and strategiesin many countries.

MSW is a heterogeneous waste, which can be divided into the following frac-tions:

– an organic biodegradable fraction, consisting of food residues and sometimesgarden wastes


Fig. 14. Average monthly biogas production from thermophilic manure based biogas plants:from Thorsoe (1994) and Blaabjerg (1996)

– a combustible fraction, consisting of an organic recalcitrant fraction, such aswood, paper and plastic

– an inert fraction, consisting of stones, glass, metal and other inorganic parts

Waste composition varies enormously, both between countries and between lo-cations in the same country. In urban built-up areas, MSW is less rich in organicbiodegradable material compared to rural areas and the composition also varieswith the social level of the area.

The organic biodegradable fraction of the MSW can be treated by anaerobicdigestion to produce energy and fertilizer. By anaerobic degradation, one ton ofthe organic fraction of MSW can give a net energy yield of 100 to 200 kWh elec-tricity plus heat.

Most often the technical aspects involved are:

– source separation and collection– separation (removal of plastic bags or other packing and foreign elements)– transfer (transporting the material through the process/plant)– heating the substrate– reactor mixing– sanitation of the biomass

Digestion of high solids wastes can take place as a continuous or batch process.Continuous processes are most desirable on a larger scale. There are many wastetreatment systems specifically designed for treatment of municipal waste (MSW).

The most treatment systems consist of the following steps:

– Pretreatment– Biological treatment– Post-treatment

4.1Pretreatment of Municipal Solid Waste

Municipal solid waste typically contains less than 1/3 organic material. In orderto treat MSW biologically, a sorting of the waste is needed. The sorting can beachieved by either source-separation, mechanical separation, or by hand sortingtechniques.

Source-separated municipal waste (SSMSW), i.e., waste sorted into frac-tions where it is generated, is the preferred solution from a quality point ofview, since it is difficult to derive a clean organic fraction once the waste has been mixed.

Such systems are found only in limited areas. However, in several places in Europe, work is being done, in order to introduce source separation of the MSW.Source-separated collection was originally introduced to derive certain valuablecomponents, due to the increased cost of raw materials and a wish to limit the useof primary resources and energy to process new raw materials.

As a consequence, source-sorted collections were introduced mainly for paper,valuable metals (such as aluminum and copper), and glass etc. Lately source-separation has also included separation of an organic degradable fraction. This


is done by including separate waste bins and collection systems in the house-holds. An alternative scheme, which has been implemented in some areas,is source separation into bags of different colors, but collected with only onetruck and later sorted out into seperate fractions by an optical sorting machine.This way the extra cost of separate collection or multi compartment collectiontrucks can be avoided (by utilizing existing collection trucks), albeit at the expense of a sorting machine.

Most of the MSW is, however, still not source-separated. Therefore, pretreat-ment is often needed to sort out the organic fraction of mixed MSW. The maingoal in the different pretreatment systems is to separate the organic fraction fromthe inorganic material. Furthermore, effort is applied in recycling several usefulcomponents, such as ferrous metals.

The fraction obtained by mechanical separation is usually more contaminatedthen the fraction obtained by source-separation, and a fraction of the organicmaterial will be lost with the other fractions sorted out. It is especially the con-tent of heavy metals and plastics that is higher in the mechanical separated MSW.Even in case of source separation, some form of pretreatment is often needed toguard the plant and end product against elements included in the organic frac-tion by mistake or carelessness.

4.2Post Treatment

Following the biological treatment, the digested material (known as digestate oreffluent) is usually dewatered to 50–55% TS with a screw press, filter press orother types of dewatering systems. The press cakes are refined with sieve andcomposted aerobically. At this point, the compost can be further cleaned byscreening, to remove unwanted material such as small pieces of glass or plastic.The compost is often offered for sale as a soil conditioner or potting soil (nitrateapprox. N = 0.25 kg/ton, total N approx. 7–8 kg/ton). Press liquid, which may con-tain high concentrations of volatile fatty acids, is centrifuged, recircled or sent towastewater treatment.

4.3Biological Treatment

There are various classification principles of the existing anaerobic systems fortreatment of high solids wastes.

There are batch and continuous systems according to the feeding procedureof the wastes to the reactors. Most systems are continuous systems. However,there also exist batch systems such as the Biocel process.

The major difference between the various systems is whether a “wet” or “dry”methanization process is used. Digestion of the MSW can take place either in themesophilic or the thermophilic temperature range, and the hydraulic retentiontime is 10 to 30 days, depending on the process temperature, the technology used,and the waste composition.


4.3.1Wet Digestion Systems

With wet digestion systems the total solids content of the waste has to be re-duced to concentrations below 20% in order to create a pumpable slurry. This can be done either by adding water, such as recycled process water, or by co-digestion where MSW is mixed with more dilute streams such as sewage sludgeor manure.

Codigestion Process

Treating high solid waste requires technically complicated and expensive treat-ment systems. Therefore, it is desirable, if possible, to avoid treating the waste asa solid, but instead mix it with other more dilute waste.

A new concept, which has been applied successfully in several biogas plants inDenmark (Vegger, Sinding Ørre, Studsgård and Århus) is mixing MSW with ma-nure. Sweden's largest biogas plant in Kristianstad work with the same co-di-gestion concept: household waste is treated together with agricultural waste andindustrial waste. The plant at Kristianstad has a capacity of 73,000 tons of bio-mass per year, of which 15% is OFMSW, 18% is industrial organic waste, and 67%is manure. The biogas yield is around 40 m3 biogas per ton feedstock and the pro-duction covers the heat requirements of 600 to 800 households. In this way someof the difficult technical problems treating high solid wastes are avoided. Alsoeconomy of scale can be achieved and the waste enters into an established nu-trient/fertilizer recycling system. The plants generally operate at thermophilictemperatures.

Another example of the excellent performance of a large-scale co-digestionprocess of the organic fraction of municipal solid waste is the sewage treatmentplant in Grindsted, Denmark where MSW is mixed with sewage sludge.

The Waasa Process

Another process using codigestion is the Waasa process. The process has beentested on a number of wastes. In the Waasa plant in the city of Vaasa, Finland thefollowing types of wastes have been treated over the years since it started up in1989; mechanically or source-separated MSW, sewage sludge, slaughterhousewaste, fish waste, and animal manure. The process operates with a TS content ofapprox. 10–15% and can operate both at mesophilic and thermophilic temper-atures.At the Waasa plant both mesophilic and thermophilic treatment methodsare in operation in two parallel reactors. Today the Waasa Process is also in op-eration in Kil, Sweden and outside Tokyo, Japan. Furthermore, a plant in Gronin-gen, the Netherlands is under construction. One characteristic of the WaasaProcess is its main reactor, which is divided into various zones in a simple way.The first zone is made up of a pre-chamber inside the main reactor.

The mixing in the reactor is by pneumatic stirring, where biogas is pumpedthrough the base of the reactor. A small part of the digestate is mixed into thenewly fed bio waste to speed up the process by inoculation.


4.3.2Dry Digestion Systems

In many cases, especially in large urban areas, it is difficult to find other wastesto codigest MSW with. Dry digestion systems can cope with solids as high as 35%.

The VALORGA Process

The Valorga process was developed in France and is a semi-dry process (Fig. 15).The process is a mesophilic process and takes place in the following way: afterpretreatment the waste is mixed with recycled process water. The process wateris gained from separation of the reactor effluent by centrifugation, filtration orother types of separation.After mixing with process water, the influent is pumpedinto the reactor. The reactor is of the fully mixed reactor type. Mixing is takingplace by pneumatic stirring, i.e., the produced biogas is compressed and sentthrough the contents of the reactor.

Only a small amount of water is recirculated, and the total solids content of thewaste in the reactor is still high. Other processes also use the same principle, i.e.,recirculation of small or large amounts of process water.

The Valorga process is a relatively widely used process. There are several full-scale plants worldwide, such as in Amiens, France (85,000 ton/year),Grenoble, France (16,000 ton/year), Tilburg, Netherlands (52,000 ton/year)(Fig. 16), Papeete, Tahiti (90,000 tons/year) and Tamara in French Polynesia(92,000 tons/year).

The DRANCO Process

The Dranco (Dry Anaerobic Composting) process is a true dry-process for treat-ment of the organic fraction of MSW (Fig. 17). Indeed, this process requires ahigh total solids content in the reactor in order to have optimal performance.


Fig. 15. Principle diagram of the Valorga process

Therefore, it is often recommended to mix non-recyclable paper or garden wasteinto the MSW in order to achieve a sufficiently high TS content. The Drancoprocess is a thermophilic process.

The process takes place in the following way: after the waste is pretreated andscreened it is mixed with recirculating material from the reactor. Three quartersof the reactor content is recycled. Mixing of the waste with this large amount ofdigested material ensures inoculation of the incoming material.

The biomass paste is pumped using piston pumps developed to pump con-crete casting mixtures.

The reactor is a downward plug-flow type reactor. In the reactor no significantmixing takes place. If the waste consists of high amounts of easily degradable


Fig. 16. View of a Valorga biogas plant (Tilburg, Holland). The plant has capacity of52,000 tonnes per year of source separated VGF (Vegetable-Garden-Fruit) waste

Fig. 17. Principle diagram of the Dranco process

material, the degradation can cause liquefaction of some of the material in the reactor, preventing the plug flow principle with the risk that some particles willflow through the reactor without achieving the required retention time. There-fore, addition of recalcitrant ligno-cellulose material is usually recommended.The digested biomass is extracted from the bottom of the reactor by a specialpatented sliding frame, pulling material evenly from the reactor cross section intoan extraction screw channel.

Today, there are several DRANCO plants in operation, Fig. 18, such as the onein Brecht, Belgium (12,000 ton/year), Salzburg,Austria (20,000 ton/year), Bassum,Germany (13,500 ton/year), and Kaiserslautern, Germany (20,000 ton/year).

The Kompogas Process

The Kompogas process is a dry process developed in Switzerland. The processoperates in the thermophilic range with a hydraulic retention time of approx.15 days. The reactor is a horizontal cylinder and the flow through the reactor isplug flow. In the reactor a stirrer provides some mixing of the waste. Recircula-tion of a part of the effluent to the incoming substrate ensures inoculation.

4.3.3Multi-stage Anaerobic Digestion Systems

Most of the digestion systems for anaerobic treatment of MSW are single-stagereactor systems. However, multi-stage digestion systems are also used. These areusually two-stage systems, although 3-stage systems have also been proposed.The idea with the multi-stage systems is to separate the different phases of theanaerobic digestion process, in order to be able to apply optimal conditions in


Fig. 18. View of a Dranco biogas plant

each of them. Different operating conditions, such as pH and retention time canbe kept in the different stages.

The BTA Process

The BTA process was developed in Germany (Kubler and Schetler 1994). Theprocess is a multi-stage process (Fig. 19). Pretreatment is centered on a hy-dro-pulper, which receives the source-sorted waste from a screw-mill, whichopens bags and disintegrates larger agglomerated particles. In the hydro-pulper,the waste is mixed with recirculated process water and the organic material is dis-solved through intense agitation. Floating light elements (plastic, wood etc.) are skimmed from the surface by a periodically operated rake and heavy items (metal, glass etc.) are removed by sedimentation. The pretreatment pro-duces a pulp of approx. 10% total solids (TS). The pulp is pumped to a buffer tankwhere acidification occurs. The effluent from the acidification reactor is dewa-tered by centrifugation. The thin liquid fraction is fed into a high flow biofilmreactor; while the thick fraction of undissolved material is mixed with processwater and fed to a CSTR reactor where further hydrolysis and acidification takesplace. The effluent from the CSTR reactor is once again dewatered, and the liquid fraction fed into the biofilm reactor for methanization at mesophilic con-ditions.

A part of the thick fraction is removed from the process to take out inert andundegradable material with a TS content of approx. 35%. This “compost”containsapprox. 0.2–0.3 kg N per ton. Excess process water, which contains most of thenitrogen (about 4–8 kg N/ton original solid waste; see below) and other nutri-ents that were in the original waste, is disposed of to the sewer system or recy-cled to the agricultural sector as a fertilizer if possible. The volatile solids (VS)destruction is predicted to approx. 85%.


Fig. 19. Principle diagram of the BTA process

4.4Summary of Processes Used for Anaerobic Treatment of Solid Wastes

There is a wide variety of configurations for anaerobic treatment of solid wastes.One way to classify these reactor systems is depicted in Fig. 20. The batch sys-

tems can be considered as accelerated landfill systems. These systems are simpleand comparatively cheap. MSW is loaded batch wise in a closed vessel contain-ing inoculum from a previous batch digestion. During the digestion period,leachate is recirculated for mixing purposes of substrate, microorganisms andmoisture.When the digestion is complete, the digested material is unloaded anda new batch digestion is initiated.

An alternative to batch digestion is the leach bed process, where the leachatefrom the base of the reactor is exchanged between established and new batchesto facilitate start up, inoculation and removal of volatile acids in the active reac-tor.When the methanogenesis is established, the reactor is connected to anotherreactor containing new MSW. This concept has also been described as sequentialbatch anaerobic composting (SEBAC).

The continuously operating systems can be divided into completely mixed andplug-flow systems. The completely mixed systems can again be classified as sys-tems based on recirculation of process water for dilution of the incoming MSWand in systems based on the codigestion concept. Codigestion is especially wellestablished in Denmark. Several systems are operating on the multi-stage diges-tion concept. However, one-stage systems are much simpler and cheaper andtherefore, considerably more widespread.


Fig. 20. Classification of anaerobic solid waste digestion systems

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Received: June 2002


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