Conservation & Recycling, Vo1.7,No.Z-4, pp.191-198, 1984 0361-3658184 $03.00/O

Printed in Great Britain Pergamon Press Ltd.

THE STATE OF THE ART REVIEW ON THE APPLICATION OF ANAEROBIC DIGESTION

CHEN RU CHEN

Macau Water Supply Company Limited, Macau

ABSTRACT

The application of anaerobic digestion expanded very slowly before 1972, but the situatimon has changed dramatically in recent years. Interest in utilising methane produced by this method has increased. Capital required for building a digester has significantly decreased because of new designs and new building materials now used. New processes for the treatment of soluble wastes have been developed which have shortened the Hydraulic Retention Time (HRT) to about 1 day. Thus, the anaerobic process can compete with the aerobic process, especially where the problem of energy shortage is crucial.

INTRODUCTION

Cesspools have been used for hundreds of years to treat night soil and manure, and to produce immediately available fertiliser. However, the application of anaerobic digestion expanded very slowly until the 1970s. The long HRT needed, which led to larger equipment, higher investment and larger areas covered, confined its adoption.

The situation has dramatically changed since 1973 when the "Energy Crisis" appeared. Interest in utilising the methane byproduct to meet the growing demand for energy has been increasing. Capital required for building digesters has been significantly reduced because of the new design and new building materials adopted. Many new processes have been developed. The most significant is the use of rapid anaerobic reactors to treat soluble wastes, including leachates of landfill refuse. The HRT in these reactors may be shortened to 1 day or less. Thus, the anaerobic process can now compete with the aerobic process, especially in the areas where the problem of energy shortage is crucial.

In China, over 5 million digesters, which are used to treat crop-stalks and manure, have been built in the rural areas since 1972[1]. India has 80,000 digesters and Brazil has 6,000. Many developing countries have their active biogas extension programmes.

The results of biogas extension are attractive. Three aspects are notable:

a) Biogas production. Biogas is used mainly for cooking and lighting. In some places it is also used for power generation, agricultural product drying and space heating.

b) Nutrient recovery. A lot of laboratory tests and field demonstrations have shown that the NPK recovery through anaerobic digestion is better than that through traditional compost.

c) Improvement of sanitary conditions.

Anaerobic digestion has been used historically to treat relatively dilute liquid wastes and suspensions with volatile solid concentrations less than 10% It was quite difficult to treat municipal solid wastes and dry agricultural residues with the traditional anaerobic digestion technology, because high expense and energy consumption were needed to grind the

191

192 C.R. Chen

solid substrates, separate inorganic materials, make slurry, stir reactor contents, and separate solid and liquid components of the digested slurry. A large scale demonstration facility capable of treating 100 tonnes day-1 of urban refuse was built in Pompano Beach, Florida, USA, and started in November 1978[21. The results seem unpromising.

A new process called "Dry Anaerobic Fermentation" has been successfully developed since the early 1970s. The process has been adopted to treat landfill refuse and agricultural resicluesC31. It was estimated that some landfills in the USA can produce 1.2 x 105m3 to 2.2 x 105m3 of biogas per day['I]. There are also some commercial landfills producing biogas in West Germany and Brazil.

A number of new anaerobic processes have been developed in recent years, but only those concerning solid waste treatment are reviewed in this paper.

IMPROVEMENT OF CONVENTIONAL PROCESSES

Anaerobic treatment of organic wastes at ambient temperature and without vigorous stirring of the digesting content is regarded as a conventional process. A long hydraulic retention time is needed when this process is adopted. This results in a large digester volume and thus a high cost for building digesters. Since conventional processes are quite simple and easy to control, it would be well accepted by farmers in developing countries, provided the cost for building a digester is not too high.

Significant improvements for building digesters have been made in China since the early 1970s. As a result, the digester cost has dropped to such a low level that most of the farmers can aford to build their own digesters. The success of biogas utilisation in China has stimulated the other developing countries to build digesters.

Now there are three types of rural digesters in practical use: water pressure type, floating cover type and membrane type.

The water pressure type was invented by Luo in the 192Osi51. The biogas produced accumulates in the gas dome (Fig.1). The biogas pressure increases with the increment of produced gas. A part of the digesting liquid content is forced out of the digesting chamber by the gas pressure, and goes up through the chamber outlet to the pressure regulation tank. A balance between gas pressure and water head is always kept automatically. When the gas valve is opened, the liquid in the regulation tank comes down to the digestion chamber, and forces the biogas out to the appliances. The gas pressure varies with the change of water head. The maximum gas pressure is usually l-1.5M water column.

Biglas

Pressure Regulation Tank /

Fig.1. Water pressure digester.

Anaerobic Digestion 193

The floating cover lifts when the biogas produced accumulates in the gas dome, and drops when the gas valve is opened (Fig.2). The gas pressure is almost constant, and equaI approx- imately to the quotient of the weight and the area of the cover.

Biogas

Fig.2. Floating cover digester.

Membrane digesters were tried in China in 1958, but they were not accepted by farmers until the 1970s. Two problems hindered their application: (i) the low thermal efficiency of the low pressure burner, and (ii) the poor durability and high gas permeability of the membrane materials. These two problems have been solved in recent years. The thermal efficiency of the burners can reacy 58-60X when the biogas pressure is as low as 2cm water column. It was reported that a new membrane material, Red Mud Plastics, which was invented in Taiwan, can last for 5-15 years under direct sunshine.

The biogas produced is stored in the space under the membrane roof or in a separate gas bag at a very low pressure, usually at O.l-2cm water column (Fig.3).

a. Digester with separate gas bag.

Fig.3. Membrane digester

b. digester with flexible membrane roof.

194 C.R. Chen

A comparison has been made between the three types of digesters[6]. The waterpressure type has not any movable parts. It can be built of common building materials, such as cement, lime, sand and stone. Most of the rural digesters are of this type. But farmers complain of the inconvenience in building and cleaning the digesters, Now it appears there is a trend to adopt membrane and floating cover types.

The membrane type is well accepted by farmers for its easiness in building and cleaning digesters, and low cost.

The floating cover type has many apparent merits, but the cost of the steel floating cover is high. A low cost floating cover made of concrete without any reinforced iron bar was designed in Chinai71. It is now widely used in Zhejiang Province.

The cost for building a family digester varies in different countries. In China, it is about US$~O-100, which is almost equal to the price of a bicycle. Farmers can afford to invest. It was reported in India that the cost of a floating cover digester is about US$200- 300.

The wastes treated by conventional anaerobic processes are mainly manure and human excrement. Cropstalks and grasses are sometimes adopted as feedstocks but the long retention time needed and low conversion efficiency limit their adoption. It was found that some energy crops, such as water hyacinth and napier grass are suitable feedstocks for conventional processes because they have a good biodegradeability.

BIOGAS RECOVERY FROM REFUSE LANDFILLS, AND DRY FERMENTATION

In the early 1970s it was discovered that anaerobic digestion proceeded in existing refuse landfills with VS concentrations as high as 25-50%. Thus, biogas could be extracted from these landfills and used for generating power. The world's first commercial landfill biogas recovery facility started in 1971 when the Los Angeles County Sanitation District constructed wells to prevent gas migration to adjacent residential areas at the Pales Verdes landfill in California (Fig.4). Since then, many other projects have been initiated in the USA, West Germany and Brazil. The Monterey Park landfill methane recovery facility is the largest one in existence and is capable of producing 2.2 x 105m3 biogas per day.

Biogas Cover

-~~-

Fig.4. Biogas recovery from landfill

Biogas production rates at existing landfills is quite low which results in high maintenance costs. In order to reduce the maintenance cost, Augenstein and co-workers initiated the study of controlled landfill in which nutrients, buffer, etc., have been added to the com- bined solid waste prior to landfilling to enhance biogas production[8,9].

Anaerobic digestion of landfill refuse is regarded as "anaerobic dry fermentation" in which the moisture content is less than 80% and the digesting materials look like solid without fluidity. The most difficult problem in dry fermentation is to keep the balance between the rate of organic acid production from refuse and the rate of its conversion to methane. If the rate of acid production is faster than the latter, acids would accumulate resulting in a drop of pH which will inhibit the methane production. The lower the moisture content, the more danger that "souring" will occur. been added as a buffer agent,

In order to avoid souring, powdered CaC03 has and sewage sludge or manure as inocul~Jm in the tests.

Anaerobic Digestion 195

Dry fermentation has been adopted to treat combined agricultural residues[lO-131. In all of the three articles from China, the authors concluded that no buffer was needed in dry fermentation provided the feedstocks had been composted for 2 to 5 days. This is of great significance to farmers who prefer to minimise the use of chemicals.

The residues after dry fermentation look like traditional compost. They are still in a solid state without fluidity, and can be carried to crop fields by shoulder poles or trans- ported by tractor drawn trailers. Thus, dry fermentation is well accepted by farmers who are used to adopting compost to treat agricultural residues. The conventional anaerobic process has been objected to by these farmers because of the difficulty in carrying dilute digested slurry to crop fields. The labour for carrying dilute slurry increases by three times or more because of the high water content.

It is impossible to adopt dry fermentation in water pressure digesters because fluidity of the digesting content is needed in these digesters to regulate the biogas pressure. The recent development of the membrane type digesters favours the adoption of dry ferment- ation. In these digesters fluidity is not necessary and the dry fermented materials can easily be cleaned.

TWO-STAGE DIGESTION

As mentioned above, it is quite difficult to keep the balance between organic acid production and its conversion to methane. In a one phase system it is impossible to create different conditions to meet the various demands of different groups of micro-organisms. To solve these problems, two-stage digestion has been tried in recent years. The first stage is called "acidification", in which the complex organic wastes are degraded to short chain organic acids, such as acetic acid, proprionic acid, etc. through the enzymatic reactions of a group of micro-organisms regarded as "acid farmers". The second stage is called "methanisation", in which the organic acids produced in the first stage are degraded through the enzymatic reactions of a group of micro-organisms regarded as "methane farmers". In each stage proper conditions favouring the activities of the respective micro-organisms are created. Thus, the retention time may be greatly shortened, and there is no danger of "souring".

Two-stage digestion to treat sewage sludge has been attempted[l4,15]. Complete mix laboratory digesters were used in both stages. The total solid retention time of the two stages is 7-7.5 days which is much shorter than that of the existing commercial contact process of 20-30 days.

Since the products of the first stage are soluble they can be treated by anaerobic rapid reactors, which are effective in treating soluble organics (see next section). Norman and ProstellCl61 tried two-stage digestion to treat dogfood with a phase-l complete-mix and a phase-2 anaerobic filter system, which is one type of rapid reactor. A high COD removal of over 93% was reached even at a total HRT as short as 3.1 days. In the landfill practice a large amount of leachate can be collected at the bottom of landfills. Methods of adopting rapid reactors to treat the leachates are being studied.

Two-stage digestion has also been used to treat soluble wastes containing toxic compounds[17] or containing mainly carbohydrates and starch because of the difficulty in pH control in a single phase system. A 130m3 demonstration two-stage digestion system for the treatment of alcohol distillate at a sugar mill in Dou Men County, Guangdong Province, China was started last September.

RAPID REACTORS

Leachates collected from the bottom of landfills and liquid products from the first stage of the two-stage digestion system are soluble. They can be treated by rapid reactors with a HRT as short as a few hours, but usually around one day.

The reason why rapid reactors have a high treatment rate and efficiency is that a high con- centration of active biomass, and thus a high concentration of active enzyme, is retained in these reactors. All types of rapid reactors are based on the principle of biomass reten- tion. They are sometimes called "Retained Biomass Reactors" or "Biomass Accumulating Reactors".

196 C.R. Chen

The anaerobic filter was the first rapid reactor developed by Young and McCarty[18]. It is an upflow filter bed with inert media (Fig.5). Bacteria grow on the media surfaces to form a fixed film or stay in the void spaces of the filter bed. A 13,250m3 fullscale anaer- obic filter, which is used to treat rum distillery slops, was started in January 1982[19].

Bicqas

m---w

. ..-.. . I . . . .,. 0. , * :*

+ * * . 1 ,__..- .\. . . ‘. * -. . * .‘.I..

_-B-M

Fig.5. Anaerobic Filter Fig.6. Anaerobic attached film expanded bed.

The fixed film concept of an anaerobic filter was extended further in the development of the anaerobic attached-film expanded bed reactor by Switzenbaum and JewellC201, in which wastes pass in an upward direction through a bed of suspended inert particles to which the bacteria are attached (Fig.6). The advantages of this reactor are the relative freedom from clogging, the much larger surfaces of the suspended media and the better contact between substrates and bacteria. Its disadvantage is the high rate of recycling required to keep the heavy media, to which the bacteria are attached, in suspension.

Upflow anaerobic sludge blanket (UASB) was invented by Lettinga in the early 19708[17,211. The characteristic of this reactor is the installation of a means of gas separation on top of the reactor which allows the bacterium floes separated from gas bubbles to settle down in the lower void spaces of the reactor (Fig.7). This prevents the bacterium floes from washing out and permits them to accumulate to form a sludge blanket, in which the concent- ration of active biomass is high. Around 20 full scale IJASB installations have been put in operation in the Netherlands, USA, West Germany, Switzerland and Austria. The largest one is a 5,000m3 unit. A lO,OOOm3 unit is now being planned.

An anaerobic baffled reactor has been studied by McCartyl221. It is a horizontal unit in which the waste passes through a zigzag route (Fig.8). The low height of the reactor limits the accelereation of the rising velocity of the gas bubbles, and the large surface area between the liquid and gas favours the bubble separation from bacteria floes. Thus, the floes tend to stay in the reactor.

Modifications of rapid reactors have been made in China since 1978. A partly filled anaerobic filter [28,24], a tube inserting anaerobic filter[25] and a horizontal pipe filterC261 have been scaled up to pilot or demonstration scales. All of these modifications are aimed at solving the inherent clogging problem of anaerobic filters and to allow better contact be- tween bacterium floes and wastes.

Anaerobic Digestion 197

Biqas

t

Fig.7. Upflow anaerobic blanket

sludge Fig.8. Anaerobic baffle reactor

Further development of rapid reactors may be the combination of different reactors. The above mentioned 130m demonstration two-stage installation in DOU Men County is a combination of the anaerobic filter and the UASB.

PLUG FLOW

Completely mixed digesters have been extensively used in sewage treatment plants. Their construction costs are expensive, and great amounts of energy are needed for continuous mixing, especially when the solid concentration of the influents are high.

In order to reduce the costs in digester construction and operation, Jewel1 and co-workers initiated the study on plug flow digestersC271. The plug flow digester is a horizontal and narrow reactor in which no mixing is required. The wastes flow by gravity from the inlet to the outlet with apparent back flow. It was reported that in adopting a low-cost, low-technology approach, the plug flow digesters have been shown to perform equally well or more effectively than the high technology, high cost completely mixed system using dairy manure as a substrateC281.

CONCLUSIONS

A significant development of anaerobic digestion technology has been achieved since the early 1970s. The construction and operation costs have been greatly reduced. The additional benefit of methane production has become more and more attractive. The HRT has been short- ened. Thus, the anaerobic digestion technology can now compete with aerobic process for waste treatment. The rapid expansion of rural biogas digesters is convincing evidence. A rapid development of dry fermentation and rapid anaerobic reactors is expected in the near future.

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2. Mooij, H.P. & Streit, M. Solid Wastes to Methane Gas, Pompano Beach, Florida, REFCON Technical Studies Report, US Dept. of Energy, Waste Management Inc. Oak Brook, Illinois (1982).

3. Wise, D.L., Low capital cost fuel gas production from combined organic residues. Tech- nical Proposal PBD-161, (AID Pre-Proposal No.3EE-20) (1982).

4. Ward, R.F., Gas Production from Sanitary Landfills (Draft). Paper prepared for UNED/TCD United Nations, pp.15 (1982).

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