anaerobic digestion of nitrophilic algal biomass from the venice lagoon

Biomass 23 (1990) 179-199

Anaerobic Digestion of Nitrophilic Algal Biomass from the Venice Lagoon

S. Rigoni-Stern, R. Rismondo

Technital S.p.A. -- v. Cattaneo 20, Verona, Italy

L. Szpyrkowicz, F. Zilio-Grandi*

University of Venice, Department of Environmental Sciences, Dorsoduro 2137, 30123 Venice, Italy

&

P. A. Vigato

Research National Center, Institute of Chemistry and Technology of Radioelements, Viale Stati Uniti, 35100 Padua, Italy

(Received 28 October 1988; revised version received 17 July 1989; accepted 25 August 1989)

ABSTRACT

The feasibility of producing biogas by anaerobic digestion of a nitrophilic algae biomass obtained from the highly eutrophicated Venice Lagoon has been investigated. Methods for harvesting algal biomass have been examined in detail and different pretreatments used prior to analysis and digestion of the algae described. Results obtained from three pilot plant digesters over a period of 12 months using Ulva rigida and Gracilaria as feed material gave no indication of inhibition of the process by either high salinity or high metals content resulting from pollutants discharged into the lagoon. Sulphides were formed during digestion as a consequence of the high sulphate content of the interstitial water as well as the level of sulphur present in the algae. However, the sulphides did not appear to cause inhibition or result in a reduction in gas yield. A maximum biogas production rate of ~347 rn 3 kg VS- i day- 1 was obtained during digestion at a retention time of 20 days with an organic loading rate of l kg VS m --~ day- i.

*To whom correspondence should be addressed. 179

Biomass 0144-4565/90/S03.50 - © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

180 S. Rigoni-Stern, R. Rismondo, L. Szpyrkowicz, F. Zilio-Grandi, P. A. Vigato

Key words: nitrophilic algae, anaerobic digestion, biogas production, digestion inhibition, Ulva rigida, Gracilaria confervoides.

INTRODUCTION

The Venice Lagoon in Northern Italy is subjected to a high level of eutrophication which results in an elevated primary and secondary production reflected in extensive algal growth. These macroalgae, which grow even when detached from the bottom, contribute significantly to pollution of the lagoon. Decomposition of algae increases oxygen demand as well as releasing nutrients and organic substances, the further metabolism of which causes anaerobic conditions leading to hydrogen sulphite production in situ. This causes not only aesthetic and olfactory problems, but also contributes further to the nutrient enrichment of the lagoon waters.

Although facilities designed to solve problems associated with discharge of domestic and industrial wastewaters have been planned these are not as yet completed. Effective nutrient elimination, the basic objective of the sewage treatment plants in the watershed area of the lagoon basin, is not expected for some considerable time. Run-off of agricultural chemicals and fertilizers, difficult to control, as well as sediments from soil erosion, contribute further to the nutrient pool.

The eutrophy of the system and the resulting problems are not easily solved. One action which has been forced upon the City Authorities is the periodic removal of algae from the lagoon. Hence, it was of interest to find an economic use for this material rather than simple, uneconomic dumping onto land.

The literature includes a number of publications coveting various direct uses of macroalgae. However, in general these consider algae harvested from the deep sea or obtained from other unpolluted areas such that products will be fit for human consumption, for use in cosmetics, for fertilizer production or for use as feedstock for the chemical industry.

Another possibility is the use of algae as feed for the production of biogas using anaerobic digestion. The anaerobic digestion of freshwater algae is well studied ~-3 although not previously examined in detail using nitrophilic lagoon algal biomass. This paper describes such experimental studies concerning the anaerobic digestion of algae carried out under varying conditions in three pilot plants over a period of 12 months. Attention was focused on problems related to biomass pretreatment in relation to biogas production rate, design parameters and inhibitory factors.

Anaerobic digestion of nitrophilic algae

THE ENVIRONMENT OF THE VENICE LAGOON AND A L G A L PRODUCTIVITY

181

The Venice Lagoon, covering approximately 55 000 ha, has a volume which generally varies between 165 and 335 million m 3 and excep- tionally reaches 510 million m 3 making it the largest saline basin of the Adriatic coast. 4 The average depth of the lagoon is only a few metres, reaching a maximum of 20 m in the navigation canal of Malamocco- Marghera. The temperature of the lagoon waters reaches a maximum (25°C) in August and a minimum (5°C) in February. The lagoon is fed by the sea through three inlets at Lido, Malamocco and Chioggia, which results in the formation of three hydraulically distinct basins. The dynamics of these hydraulic regions reflect the influence of the sea tides as well as the influence of a whole network of natural and artificial canals of different depths.

A direct inflow of fresh waters, from small water courses and water run-off from adjacent land masses, contributes to the overall hydraulic system -- but to a lesser extent than the tidal system. Rivers flowing directly into the lagoon contribute to changes in water salinity only locally and do not have a great effect on the lagoon as a whole. In contrast, the salinity of the water which flows into the lagoon from the sea is influenced by rivers opening in the north-west region of the Adriatic sea. As a result, the Venice Lagoon has regions of varying salinity, although in most of the basin the salt concentration is very close to that typical of marine coastal waters. Due to domestic and industrial discharges as well as inflow from an irrigation network, the lagoon is polluted by metabolites, toxins and nutrients (see Table 1 ).5

Although the concentrations of nitrogen and phosPhorus in the Venice Lagoon vary significantly from time to time and from region to region, values are always relatively high. Phosphorus is present at concentrations of up to 30 pg litre-~ whilst concentration of ammonium ions may reach levels of several hundred/~g litre-1. The highest values are found near the domestic and industrial discharge points and in the zones of agricultural run-off. Minimum levels occur during periods of algal bloom between March and July. 6

As a consequence of the high nutrient availability and other favour- able environmental conditions, particularly high temperature and insolation, the productivity of the lagoon is very high, reflecting average rates of carbon fixation of around 280 mg C m -2 day-1. Maximum values of 1000 mg m- 2 day- 1 can be observed in spring and late summer with minimum rates of around 100 mg m -2 day- l in February. 7 These rates of carbon assimilation are the consequence of both periodic


phytoplancton blooms and the growth of macroalgae species with high nitrogen requirements.

The gradual displacement of the Phanerogame group typical of salt lagoons, as well as various species of algae which require a less polluted environment for their growth, 8 has left a standing crop of over 1 million tons 9 composed of only a few species dominated by Ulva rigida. At the same time the increased availability of organic matter and establishment of anoxic conditions in the lagoon favours a huge proliferation of Chironomids.

Of the various algal species which develop in the Venice Lagoon, those which have industrial value include: Gracilaria confervoides, Ulva rigida, Chaetomorpha aerea and Valonia aegaegropila. Gracilaria confervoides and Chaetomorpha aerea grow as a filamentous mass in the form of hanks. Some varieties of Ulva rigida develop in a laminar form on the surface where a single frond may cover several square metres. Valonia aegaegropila forms spheres of 15-20 cm of diameter giving it a grape-like appearance.

The average concentration of the various species of algae in the different basins are summarized in Table 2. The central lagoon basin is the most eutrophic zone as a consequence of domestic sewage discharge

TABLE 1 Estimates of Phosphorus and Nitrogen Inputs into the Venice Lagoon (t year- 1)

Nutrien~ Domestic Industrial Agricultural Atmosphedc Toml sewage effluen~ run-off faH-out

N 2680 1500 5200 560 9940 P 1150 700 900 85 2835

TABLE 2 Concentrations of Various Algae Species Found in the Venice Lagoon

Region Predominant algal species

Concentration of algae (kg m- 2)

Average Maximum

South Basin Central Basin North Basin

Ulva rigida 5 10 Ulva rigida 5 45 Valonia > 10 Ulva + Gracilaria 1

Anaerobic digestion of nitrophilic algae 183

from Venice and the Lido. Here, algal biomass concentration, with Ulva rigida predominating, averages about 5 kg m-2. The algae grow as deep as 0.8-1.2 m, reaching the water surface and sometimes emerging from it. Once exposed in this way, the rate of degradation of the algae is very rapid, characterized by colour loss. In the south basin, extending over 25 km 2, the predominant species Ulva rigida can reach levels of 10 kg m-2. In the north basin, Valonia aegaegropila predominates quantitatively, although Ulva and Gracilaria are also found. However, Valonia is only found in the detached form suggesting that it comes from other zones. In some places it may accumulate to concentrations which reach values higher than 10 kg m -2. Ulva and Gracilaria are more uniformly dis- tributed throughout the region at concentrations of up to 1 kg m- 2.

SUBSTRATES

Harvesting and storage of algal substrates

Algae which become detached from the bottom accumulate near natural or artificial barriers (e.g. mussels fishing piling cages) or in depression zones. The majority of these sites are characterized by shallow waters (maximum depth of 2 m), which facilitate algae harvesting during low tides. Algae were harvested from boats in such areas of accumulation using rakes or nets. For the purpose of this study about 900 kg of algae (mainly Ulva and Gracilaria) were collected. Since the algal biomass is present all year, the amount stored was limited to that required to run the plant for 1 or 2 days.

Substrate pretreatment

Most digestion tests were conducted using algal mixtures which were proportionally the same as in the harvest material: 80-90% of Ulva and 10-20% Gracilaria since it was difficult to separate these two species during substrate pretreatment. The algae were either water washed or only drained. The total and volatile solids content of drained raw or washed algae are shown in Table 3. The washing procedure involved the immersion of the algal biomass in a volume of fresh water double that of the algae. Hurdle bottom (2 x i m) containers of 0"25 m height were loaded with a 10-cm layer of algae, immersed in three consecutive baths for 3 min and racked. Washing removed soil and as a consequence the volatile solids content of the algae increased. The characteristics of the

184 S. Rigoni-Stern, R. Rismondo, L. Szpyrkowiez, F. Zilio-Grandi, P. A. Vigato

wash-water are shown in Table 4. The amount of solids removed by washing in this way was higher in the case of Ulva due to its higher surface area and its natural tendency to form wrinkles and to retain different material.

Prior to the digestion, algae were homogenized to facilitate pumping and to increase digestibility. Various methods of crushing algae were investigated, with the objective of obtaining maximum cell wall destruction and volatile solids release from the biomass. Initially a lamellae mill was used with algae dried in ambient air. The material was reduced to give irregular pieces smaller than 2 mm. The 'flour' so obtained, with a water content reduced to the range of 6-9%, could be preserved in plastic containers without deterioration.

Microscopic examination of milled material indicated that such simple mechanical treatment did not break cell walls. Further tests showed that during anaerobic digestion bacterial lysis of cell walls did not occur and furthermore, that a significant part of the cell material remained intact at the end of the process. In order to increase the degree of disruption of the tissue, various pressing techniques were also investigated. Algae (drained and not washed, with total and volatile solids contents of 15 and 64%, respectively) were submitted to a pressure of 30.4 × 106 N m -2 resulting in 7 litres of juice from every 10 kg of biomass (see Table 5).

TABLE 3 Total and Volatile Solids Content of Algae Collected from the Venice Lagoon, Values

Averaged from Eight Samples

Algal species Total solids (%) Volatile solids (%)

Raw Washed Raw Washed

Ulvarigida 16.3+1 '21 12'8+0"51 60.1+10"1 81"2+4"71 Gracilariaconfervoides 18.4+1.12 17.4+0.57 76 '5+2"04 85.1+1"02

TABLE 4 Properties of Wash-Water Obtained from Samples of Algae

from the Venice Lagoon

Sedimentable solids (ml litre -~ ) Suspended solids (g litre- ~) Conductivity ( p S) Total solids (%) Sulphates (mg litre- ~)

11"75 1"79

2237'5 68"4 96"5


Examination of the expressed liquid under an optical microscope (Zeiss III) showed an insignificant presence of endocellular material (chloroplasts). Samples of the solid residue, resuspended in water, examined in the same way, showed about 10% cell breakage. A further sample of drained algae (with a total and volatile solids content of 16 and 61%, respectively) was squeezed four times at 100 × 106 N m-2 pressure (see Table 6). In this case 7"25 litres of juice were obtained from 10 kg of algae. As indicated in Table 6 only 26% of volatile solids present initially in the algae were found in the juices after four pressings.

The extraction yield of sequential pressings dropped from 20% of VS for the first one to 4% for the second, indicating that it was pointless to extend the treatment to more than two pressings. Again microscopic examination showed a clear juice whilst in this case 15-20% of the algal cells inthe residue were empty with broken walls.

The effects of milling the algae were also investigated. Drained algae were first hammer-milled and then homogenized in an industrial apparatus (Loro-Parisini attrition mill). This combined process led to almost complete cell breakage. Table 7 reports the characteristics of the homogenized-milled mass in which all the volatile solids, initially

TABLE 5 Properties of Juice Obtained by Pressing Algae from the Venice Lagoon at

30.4× 106Nm --~

Total solids (g litre- i) Volatile solids (%) Suspended solids (g litre ~) Sedimentable solids (ml litre-t) COD of raw juice (mg litre- i) COD after filtration on 0"45/~ membrane (mg litre t)

34"4 36"5 14"7

138'0 9080 5570

TABLE 6 Properties of Juice Obtained by Pressing Algae from the Venice Lagoon at 101 × 10 6 N

m-2

Pressing 1 2 3 4

Total solids (g litre- ~) 108"30 22"43 2'63 2'61 Volatile solids (g litre- ~ ) 31-44 7.12 1.08 1.08 Sedimentable solids (ml litre- ~) 616'00 n.d. n.d. n.d. Suspended solids (g litre- 1) 91.50 17.17 0"50 0.43 Volatile susp. solids (g litre- ~) 14.54 2.98 0-26 0.25 COD of raw juice (g litre- ~) 27"68 4.67 1.43 1.32 COD after filtration on 0.45/~ membrane (g litre- ~) 3"62 1.29 0"69 0.64


present in the algae, were retained. Microscopic examination showed a mixture of cell contents and cell wall pieces, untouched cells were rare.

MATERIALS AND METHODS

Inocula

Natural freshwater inocula were used rather than material from saline environments (bottom sediments 1°, 11) in order to avoid problems due to differences in osmotic pressures. 12-14 The inocula were obtained from an anaerobic digester sited in a domestic sewage treatment plant (inoculum A) and from a laboratory digester adapted to lagoon algae (inoculum B). Both inocula, when examined under a light microscope, showed a wide bacterial spectrum. A bacterial count carried out using Gibbs and Freame medium ~5 indicated that inoculum A had a bacterial concentration of 10 9 per ml with a predominance of spherical micro-organisms whilst inoculum B contained 10 7 cells per ml with the equal quantities of spherical and rod-shaped bacteria.

Preliminary tests of inhibition phenomena

Preliminary tests were carried out using a laboratory digester of 20 litres volume, in order to evaluate the extent of inhibition of the digestion process by various factors such as intrinsic salinity of the algae (for both drained and washed material), heavy metals c o n t e n t 16,17 and high nitrogen content.L8 Using algae washed in tap-water the process of the anaerobic digestion started up relatively quickly and biogas production occurred, indicating a lack of the inhibition by salinity, in contrast to previous reports.~ l, 12, 19, 20

The high sulphur content of the algae (analysis of average samples obtained by combining material from several sites indicated sulphur contents equal to about 3%, to compare with a literature value of 1-5%) as well as that present as sulphate in the algae inbibition water (reflecting the sulphate concentration of the lagoon water at around 2000 mg

TABLE 7 Properties of Homogenized and Milled Algae

Total solids (%) 8-5 to 13 Volatile solids (%) 43 to 62 COD (g l i tre- 1) 44 to 50


litre-l) did not apparently cause problems in digestion once the reactor was acclimatized to this substrate. During the tests the sulphur content of the algae was converted into sulphides at a concentration in the range of 300-500 mg litre- 1.

Further tests were carried out using U. rigida collected in the lagoon in the proximity of a domestic and industrial solid wastes disposal site to check the possibility of zinc and iron inhibition. 21 Data obtained with atomic absorption and X-ray fluorescence analysis (Table 8) did not indicate high contamination levels of the algae. In fact no problems from heavy metals were observed; it is quite probable that the greater part of any such metal contamination would have become immobilized as sulphide during digestion as a consequence of the high sulphur level.

Again in contrast to some literature reports 18 an elevated nitrogen content did not produce any observable toxicity effects. Most of the nitrogen present was eliminated from the digester, either in the sludge or in the effluent.

Anaerobic digestion experiments

Digestion tests were carried out using three pilot plants of 180 litres capacity with the following characteristics:

Digester D-1: constructed of plastic 5 mm thick, insulated with glass wool layer of 50 mm thick, fed by peristaltic pump operated by a timer with biomass mixing accomplished by external recycling using a centrifugal pump of 80 litres min- r flow rate Digester D-2: built in stainless steel 1.5 mm thick, other characteristics as D-1

TABLE 8 Heavy Metals Content of Washed Algae from the Venice Lagoon. Results Expressed as % of Dried Substance from

Samples with a Volatile Solids Content of 75%

Sample 1 Sample 2

Copper 0.0039 0"0027 Chromium 0-0073 0-0012 Zinc 0.029 0"047 Nickel 0-097 Traces Aluminium 0-37 0"27 Iron 0-54 0"405 Lead Absent Traces


Digester D-3: as D-2, provided with a final clarifier of 12 litres volume with sludge recycled from the clarifier using a peristaltic pump of variable flow rate.

Digestion was carried out under mesophilic conditions (35°C+1) following inoculation with 20 litres of sludge. The digesters were fed initially with washed, dried and comminuted algae at volumetric loading rates of around 0.2 kg VS m -3 day-~, which was estimated as 5-10% of the optimum loading rate for process stability. The subsequent increase in loading rate was adjusted to compensate for the low initial bacterial level of around 5% TS. Once steady state had been reached, the volumetric loads were gradually changed over a period of about a month, from the initial rate to between 2 and 4 kg VS m- 3 day- ~. The conditions required for steady state in terms of microbial biomass concentration were monitored in relation to various volumetric loading rates and feeding regimes. Two alternative loading procedures were used: semi- continuous or twice a day. The semi-continuous method was used with algal suspensions loaded by a pump running for 3 min in every 20 min. Alternatively the digesters were fed washed, dried, comminuted algae twice a day.

Sludge was only recycled in experiments using digester D-3. For most of the tests the hydraulic recycling rate was maintained equal to the feeding rate. The digesters were used over a period of 11 months, during which the experimental conditions were varied as discussed below.

Analysis

Analytical procedures and determinations of biological parameters were carried out using Standard Methods. 22 Gas was stored in plastified PVC bag gasometers of 300 litres. Gas volume was measured using a wet counter, equipped with a water manometer and a thermometer. All gas measurements are expressed as STP. The composition of the biogas produced was determined using a Hewlett Packard gas chromatograph (model 5070), equipped with a thermoconductivity detector and a Chromosorb W column.

DIGESTION EXPERIMENTS

Digester D- 1

In the first series of experiments the digester was fed semi-continuously with washed, dried and comminuted algae after seeding with inoculum

Anaerobic digestion of nitrophific algae 189

A. After acclimatization the digester was fed at an organic loading rate (OLR) of 2 kg VS m -3 day -1, maintaining a HRT of 20 days. After 2 months a decrease in gas production was observed coupled to an increase in the ratio of volatile acids to alkalinity whilst the methane content dropped to 36% and the hydrogen sulphide reached 6"5%. Reducing the OLR to 1 kg VS m - 3 day- 1 resulted in a new steady state being reached quite quickly, with a maximum biogas production of 62.5 litres day -1 (0.347 m 3 of biogas per 1 kg of VS introduced into the digester). The process parameters and analytical data determined during this stable phase are reported in Table 9.

During this period the average methane content of the biogas was 61 + 2-9% whilst hydrogen sulphide was as low as 0.67 + 0.13%. The mean total solids reduction was about 44% and that of volatile solids 62.5 + 1.3%. These values were consistent with values expected from the digestion of the normal plant substrates. 23 However, in spite of the apparent satisfactory operation of the digester, problems arose from a high concentration of sulphides in the surnatant (mean value 250 mg litre- ~ ). Addition of 100 g of hydrated ferric chloride over a period of 5 days every week reduced the sulphide levels to around 50-60 mg litre-~. However, the levels subsequently increased back to the initial values and further increase in the chloride dose did not result in significant improvement.

A second series of experiments was carried out using unwashed algae as substrate, after air-drying to a moisture content of 7 -10% giving a VS content of 47-61%. At a hydraulic retention time (HRT) of 15 days biogas production was 0-291 m 3 per kg of volatile solids with a gas composition similar to that indicated above with a mean methane content 62.6 + 2.5%. The average destruction of VS was 52.5 + 1.43%.

For a third run the D-1 digester ws fed with the juice from homogenized algae with a HRT of 20 days. The content of total and volatile solids in the juice was adjusted to match that of the previous run using unwashed, comminuted algae. In this test a gradual increase in biogas production was observed (Fig. 1), from a value of 0.291 m 3 per kg VS (methane content 63%) to 0.322 m 3 per kg VS (methane content 70%) which became stable after about 35-40 days from the beginning of the run.

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(mg

litr

e- l

)

21 +

2"0

6 21

"7 +

7-6

42

"0 _

+ 3" 1

13

"7 +

4"0

7-1

+0-

22

16"9

+8"

2 15

"0-+

0"3

7"3+

1"8

143

32

1+

60

-9

51

5+

50

"0

19

5+

11

6

10

46

0

38

83

0-+

15

67

8

55

0+

56

50

62

57

+8

23

1

40

43

+2

98

4

13

27

2+

73

1

70

30

+1

22

32

5 +

10'8

5

98

+4

7

58

3+

15

38

4+

21

6

90

+ 1

6"3

1 0

50

+3

2

164

-+ 3

5"8

315

+ 36

22

4 +

36

273

_+ 3

0

56

50

_+

50

0

15

'5+

0-4

7"5+

0"14

388

+ 31

"9

90

30

+

160

491

+ 16

95

5 +

101

192

+ 8"

6

19

0+

10

1-25

+ 0

"25

197+

7"5

1"1

+0"

2 2-

7 +

0-2

2 97

3 +

103

890

+ 20

N


as previously described. The HRT was changed to 15 days and the organic load maintained at 2 kg VS m- 3 day- ~ with feeding twice a day. Data from the stable phase of this run are given in Tables 9 and 10 whilst changes in other parameters with time are illustrated in Fig. 3.

The digester was then fed semi-continuously with the juice of the same type as used with digester D-1 phase but adjusting the VS content to match that of the algae previously fed to this digester and maintaining the HRT at 20 days. Total biogas production increased as compared with digestion of whole algae and the methane content was higher. The biogas yield increased from 0-22 m 3 per kg VS (58% methane) to 0.287 m 3 (67% methane)(Fig. 4).

Digester D-3

The digester D-3 was seeded using inoculum A and initially fed semi- continuously with washed, dried and comminuted algae at an OLR of 0.2 kg VS m- 3 day- ~ with sludge recyling in the start-up phase equal to 50% of the inlet flow. Over 30 days the OLR was gradually increased to 4 kg VS m- 3 day -~ ~ at a HRT of 30 days. Under these conditions the biogas production rate was low. Hence, after 60 days of operation the OLR was reduced to 2 kg VS m- 3 day- ~ and the HRT changed to 10 days with the feed pump running for 3 rain every 12 rain and a recycle rate equal to the

> ,

1D 0 . 4 2.0

~ 0"3 1-5

z 0.2 .~1.0 Ic _o

£ o. 0.1. 0.5

m

0

Fig. 2.

80 L

- - Biogas . . . . . . . C H 4 1 C O 2 . . . . . . Alkal in i ty

I I I I I 1 I 0 8 16 24 32 4 0 4 8 56

T ime (days)

Performance of the digester D-2 fed with washed algae at an organic loading rate of 2 kg VS m - 3 day- 1 with a hydraulic retention time of 20 days.

2 . 0 -


inflow rate. Under these conditions the sludge concentration in the digester averaged 40-50 kg m-3. However, a significant loss of sludge in the effluent was noticed, probably caused by gas flotation in the clarifier.

"~ 0.4 20 8.0

" 0

®

~ 0.3 1.5

m k) E - z 0.2 g l.0 { . . -

~0.1 0.5

m

0 0

Fig. 3.

~ 6.0

~4.0

2.0

0

/

Biogas . . . . . . . C H 4 1 C O 2 . . . . . . Alkalinity

I I I I I I I 8 16 24 32 56 64 72 80

I I 40 48

Time (days)

Performance of the digester D-2 fed with unwashed algae at an organic loading rate of 2 kg VS m - 3 day- 1 with a hydraulic tenteion time of 15 days.

0.4 Q. -g

~ 0.3 O ) v .

E z 0 . 2 ._o ~d

cL o.1

._~ m

Fig. 4.

2.C

1.5

)

il-C

0.51

C ~ 6.0

~4 .0

2.0

t IX /l/,t~ \\\¢ III l\ \\ //XX\ ii \\ \ l X

d / i /

m

- - Biogas . . . . . . . CH4/CO 2 . . . . . . Alkalinity

o l I I I I I I I 0 8 16 24 32 40 48 56

Time (days)

Performance of the digester D-2 fed with homogenized algae at an organic loading rate of 2 kg VS m- 3 day- 1 with a hydraulic retention time of 20 days.


Flows were adjusted to bring back the suspended solids to values similar to those observed in the digester D-1 and the digester D-3 operated at similar loading rates as initially used with digester D-1. This was done with the objective of verifying the data previously obtained as well as to evaluate any differences which might appear after a long period of functioning with the same substrate under the same conditions. Results are shown in Table 9 and Fig. 5.

DISCUSSION

In general these tests established that the macroalgae from the Venice Lagoon could be treated by anaerobic digestion without any long-lasting serious problems associated with the intrinsic characteristics of the algae. It was anticipated that under mesophilic conditions some inhibition due to sodium and sulphate would occur. However, preliminary tests sug- gested that toxic levels are not reached when washed algae were used as substrate. In the initial tests ferric salts were investigated as a means of reducing the concentration of free sulphides. However, such problems gradually diminished during start-up as the process stabilized. Even with sulphide contents in the digester biomass as high as 180-350 mg litre- inhibition of biogas production or alteration of its composition did not occur once steady state had been reached.

~0.4 t'~

"tJ

0.3 >

0-2 .o

"o

2 ~ 0.1

m

Fig. 5.

2-0 t 8.C

P3

1.5 ~6-C

~ =

0.5[ 2.C

. . . . . . . . I

/ OL I I I I I I I I I I

0 48 56 64 72 80 88 96 104 112 120 Time (days)

- - Biogas . . . . . . . . CH4/CO 2 . . . . . . . Alkalinity I I I I I

8 16 24 32 40

Performance of the digester D-3 fed with washed algae at an organic loading rate of I kg VS m - 3 day- t with a hydraulic retention time of 20 days.


No inhibition could be associated with the presence of heavy metals (AI, Fe, Cu and Zn), possibly due to the fact that reaction with the high levels of sulphur derivatives resulted in most of the metals being incorporated into the sludge in an insoluble form. In fact, the metals content of the settled sludge was always found to be higher than that of the supernatant.

The best experimental results obtained from the three digesters, using different feeds, are summarized in Table 11. Organic load reduction, expressed as volatile solids destruction, was around 60%, which can be considered as a normal value for the anaerobic digestion of vegetable, algal or other substrates. A HRT of between 15 and 20 days can be regarded as optimal for intact algae. The results indicate that under the conditions investigated here the HRT cannot be reduced below such times without causing a noticeable drop in biogas production: 25-30% for a HRT of less than 15 days. Digestion of juice from unwashed algae gave yields of biogas which were only slightly higher than those observed with washed, dried and comminuted algae at the same HRT of 20 days. However, the methane content of the gas was higher.

The total methane production, in terms of yield on VS fed, was the same irrespective of pretreatment. Hence, the choice of any pretreatment depends on the assumed cost benefit and not on any increase in process efficiency. During the treatment of unwashed algae the yield decreased by 30% in comparison with the digestion of washed algae with OLR, but a longer HRT. At the same time, the reduction in VS dropped from around 63% to around 50%. As far as recycling of the active biomass is concerned, insufficient sedimentation and the consequent significant biomass loss from the outflow, made this alternative less interesting, even though biogas yields as well as the methane and the sulphur content of the biogas are satisfactory.

The possibility of digesting algal biomass characterized by 50-60% of an organic content in the form of VS is reflected in the cellular composition, with a residual material refractory to digestion (40-50% of the total organic content) consisting of cells walls. A simple mechanical com- minution, however carefully carried out, did not result in significant cell separation nor did it result in total shredding of the cell walls. The algal species with thinner cells and a simpler anatomical structure, such as Ulva, were easier to disrupt and digest. On the other hand, species with stiff outer walls, such as Valonia, which are difficult to comminute were not found easy to digest in these laboratory-scale experiments and similar behaviour can be expected in a large-scale digester.

Gracilaria confervoides, present to a lesser extent in the algal biomass utilized here, falls into an intermediate category between the two

TA

BL

E 1

1 S

umm

ary

of B

est

Rea

ctor

Per

form

ance

Par

amet

ers

Subs

trat

e L

oa

d

Ret

enti

on

(kg

VS

m-

-~ d

ay-

i)

tim

e ~d

ays)

Bio

gas

prod

ucti

on

(m 3

kg V

S- 1

day

- i)

Met

hane

co

nten

t V

S re

duct

ion

(%)

Was

hed

drie

d co

mm

inut

ed

alga

l, pi

eces

<

2 r

am,

Ulv

a 8

0-9

0%

G

raci

lari

a 2

0-1

0%

Raw

alg

ae, w

ith s

and

and

lime,

dr

ied

and

com

min

uted

, pi

eces

< 2

mm

, U

lva

80-9

0%,

Gra

cila

ria

20

-10

%

Raw

alg

ae, w

ith s

and

and

lime,

m

illed

and

hom

ogen

ized

, pi

eces

< 0

-5 m

m,

Ulv

a 80

-90%

, G

raci

lari

a 2

0-1

0%

1 20

0"

347

61

63

2 20

0"

316

56

63

1 15

0"

322

63

54

2 15

0-

222

58

50

1 20

0"

291

70

60

2 20

0"

277

67

57

7=

198 s. Rigoni-Stern, R. Rismondo, L. Szpyrkowicz, F. Zilio-Grandi, P. A. Vigato

extremes typified by Ulva and Valonia. It would appear that in the natural environment of the lagoon and other seaside zones characterized by a massive nitrophilic algae growth, a well-developed process of cellulose and hemicellulose destruction exists. This conclusion is based on the fact that during microscopic examination of the lagoon sediments a significant accumulation of cellular wall material from algae was not found. This is probably the consequence of aerobical processes of cellulose destruction. 5

CONCLUSIONS

These studies confirmed the possibility of using nitrophilic macroalgae of Venice Lagoon as the substrate for biogas production by conventional anaerobic digestion. Some problems were found, particularly those relating to the shredding of algae to allow efficient digestion through enhanced release of endocellular material. There is still a need to find a suitable pretreatment which would enhance the digestion of the cell wall material. However, in spite of the difficulties in digesting the cellulosic material, anaerobic digestion of the algal substrates was found to proceed at a velocity and efficiency comparable with those observed during the digestion of secondary biological sludge in conventional digestion systems. A mixture of Ulva rigida and Gracilaria confervoides proved to be the best substrates.

ACKNOWLEDGEMENTS

This work was supported in part by the National Research Center of Italy and in part by the Commission of the European Communities.

REFERENCES

1. Levring, T. H., Hoppe, H. A. & Schmid, O. J., Marine Algae. A Survey of Research and Utilization. Cram, De Gruyter, Hamburg, 1969.

2. Hamisak, M. D., Recycling the residue from anaerobic digester as a nutrients source for seaweed growth. Bot. Mar., 24 (1981 ) 57-61.

3. Dawes, C. J., Marine Botany. John Wiley, New York, 1981. 4. Croatto, U., Utilizzazione di macroalghe della laguna di Venezia nella

produzione di biogas e riduzione dei fattori di eutrofizzazione. Conv. Int. 'Fitodepurazione e impieghi delle biomasse prodotte', Parma, 1981.


5. Rismondo, A., I1 problema della degradazione microbica della cellulosa di origine algale nella laguna di Venezia: tentativi di interpretazione e quantificazione. Universita' degli Studi di Padova, Tesi di Laurea.

6. Zingales, F., Marani, A., Benduricchio, G. & Rinaldo, Nonpoint source pollution of the Venice Lagoon: perspectives of long-term abatement. In Proceedings of Nonpoint Pollution Abatement Symposium, Milwaukee, WI, 1985.

7. Croatto, U., Biogas da macroalghe lagunari. Conferenza Internazionale su Energia da Biomasse, 1982.

8. Round, F. E., The Ecology of Algae. Cambridge University Press, Cambridge, UK, 1981.

9. Solazzi, A., Produttivita' primaria in Mediterraneo. Mem. Biol. Marina Oecianogr., 4 (1974) 101-20.

10. ZoBell, C. E., Marine Microbiology -- A Monograph on Hydrobacteriology. Cronica Botanica, Waltham Man, 1946.

11. Brisou, J., Microbiologie du Milieu Marin. Collection de l'Institut Pasteur, Flammanon, Paris, 1955.

12. Ludzack, F. J. & Noran, D. K., Tolerance of High Salinities by Conventional Wastewater Treatment Process, 37th Annual Conference of WPCF, Bal Harbour, FL, 27 September-1 October 1964.

13. Gilli, G. & Comune, P. M., Effetti dell'alta salinita' sui batteri. Inquina- mento, 10 (1980) 31-5.

14. Hockembury, M. R., Burstein, D. & Jamro, E. S., Total Dissolved Solid Effects on Biological Treatment, 32nd Industrial Waste Conference, Purdue University, 1977.

15. Gibbs, B. M. & Freame, B., Methods for the recovery of clostridia from foods. J. Appl. Bacteriol., 28 (1965) 95-111.

16. Perin, G., Inquinamento chimico della laguna di Venezia, Atti Conv. 'Problemi dell'inquinamento lagunare', Cons. Dep. Acque Zona Industriale di Porto Marghera, Venezia, 1975.

17. Perin, G., Rapporto preliminare sui metalli nella laguna di Venezia, Atti III ° Conv. Naz. 'Risorse ed Ambiente', Bressanone, 1975.

18. McCarty, P. L., Anaerobic Waste Treatment Fundamentals/3 -- Toxic Materials and Their Control. Publ. Works 95 (1964) 123-6.

19. McCarty, P. L. & McKinney, R. E., Salt toxicity in anaerobic digestion. JWPCF, 33 (1961) 4, 339.

20. Kugelman, I. J. & McCarty, P. I., Cation toxicity and stimulation in anaerobic waste treatment. JWPCF, 37 (1965) 1, 97.

21. Karaszewska Szpyrkowicz, L. & Orio, A., Modello di regressione lineare della distribuzione di metalli pesanti in sedimenti siltosi. Ingegneria Ambientale, 14 (1985) 5,255-60.

22. APHA (American Public Health Association), Standard Methods for Examination of Water and Wastewater, 14th edn. APHA, AWWA, WPCF, Washington, DC, 1975.

23. Speace, R. E. & McCarty, P. L., Nutrient requirements and biological solids accumulation in anaerobic digestion. Advanes in Water Pollution Research, Proc. Int. Conf., Vol. H. Pergamon Press, Oxford, 1962, pp. 305-22.

anaerobic digestion of nitrophilic algal biomass from the venice lagoon

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