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243SUMMARY AND CONCLUSIONS

An experimental facility was built to test the feasibility of recovering nutrients andenergy from swine manure. A source of industrial waste heat was simulated to enhance thebiological processes involved. Algae and bacteria were used to convert nutrients into biomassand to generate biogas.

Manure was removed from the animal quarters by a gutter flushing system and solids wereseparated from liquids by gravity settling. Solids were subjected to anaerobic digestion in a14 m3 digester to release plant nutrients, reduce the volume of the solids, stabilize the manurebiologically to reduce odor problems, and recover energy in the form of methane gas.

The liquid phase of the manure was used as a nutrient substrate for the growth of algaeand bacteria in 12 outdoor basins with a combined surface area of 24 m 2 and a combined vol-ume of 6,000 I.

The waste heat needed to maintain above-ambient temperatures in the bioconversionunits was simulated by heating water in heat exchangers with electrical resistance heaters. Anaccurate heat balance for digester and basins was obtained by using kWh meters.

The experimental facility was designed so that the waste water could either be recycledfor flushing or discharged into a neaby lagoon. In the latter case, only fresh water was used forflushing of the animal quarters.

The experimental facility was used to study:1. the functioning of the digester and the algal growth basins,2 . the degree of nutrient removal,3 . dry matter and protein yields,4. production of methane gas,5. the energy requirements of the digester and the outdoor basins,6. the quality of the waste water after removal of the biomass,7. the biological stability of the conversion units,8. the practical and economical feasibility of the nutrient recovery system,9. the development of design criteria for a full-scale operation, and

10. the formulation of other bioconversion processes which might improve the recoveryof nutrients as well as the quality and quantity of end products from swine manure.

Growth of AlgaeIn early laboratory tests, the high temperature strain 211/8K of Ch/orella vulgaris was

found to grow better in swine waste than Scenedesmus obliquus Sc D3WT, Scenedesmusquadricauda, Selenastrum capricornutum, Spirulina major, and Botyrococcus, and was there-fore selected for use in the field experiments.

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244Experiments were designed to test the ability of Chlorella vulgaris 211/8K to establish

itself as the predominant algal species and to find the combinations of culture depth, retentiontime, and temperature which produce the highest cell concentrations under prevailing climaticconditions.

It was determined that the fresh manure must be diluted before the liquid phase is suit-able for algal growth. In terms of inorganic nitrogen the manure must be diluted to contain150 to 250 mg of elemental nitrogen in the ammonium form per liter. This is achieved by a50-fold dilution of the volume of manure excreted by the animals, including liquids and solids.When the manure is not sufficiently diluted, the high concentration of organic matterpromotes bacterial growth over algal growth.

Light RequirementsIt was found that Chlorella vulgaris 211/8K requires a monthly average of the daily rate

of solar radiation in excess of 2,000 kcal/m 2 day and a photoperiod of at least 11 h, in orderto maintain a sufficiently high concentration of cells for harvesting at retention times of 8 daysor less. Growth continues at less than 2,000 kcal/m 2 day but at a reduced rate, so that adjust-ments in the retention time and/or culture depth are necessary to prevent depletion of thealgae and eventual predominance of bacterial growth.

Adjustments in retention time and/or culture depth are also necessary whenever a pro-longed period of cloudiness occurs during which the rate of solar radiation remains below2,000 kcal/m2 day, although the monthly average of the daily rate may still be above 2,000kcal/m2 day.

Culture DepthThe highest concentrations of algae were found at the culture depth of 10 cm. Increasing

the culture depth to 20 cm reduced the concentration of algae by 50 percent. However, thetotal biomass, including algae and bacteria, was nearly the same at either depth.

TemperatureThe use of waste heat to maintain a culture temperature of at least 25 C was essential in

achieving high concentrations of algae, except during the summer months when the monthlyaverage of the daily maximum air temperature was 25 C and the monthly average of the dailyminimum air temperature was not less than 10 C. The statement assumes continuous mixingof the algae and equilibrium of temperature between the culture medium and air.

Maximum concentrations of Chlorella were obtained at 30 C or 35 C. However, the con-tribution made by the algae to the total biomass, including bacteria, was higher at the tempera-ture of 35 C than at 30 C. This was attributed to the production of anti-bacterial substancesby Chlorella at 35 C but not at 30 C.

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245Retention Time

The concentration of algal cells was found to increase with retention time. Maximumconcentrations were obtained at retention times of 6 to 8 days, indicating a generation time ofCh/ore//a of 72 to 96 h under field conditions.

Concentrations of Algal CellsConcentrations of cells of Ch/ore//a greater than 107 /ml could be maintained in theproperly diluted waste water at retention times of 3 to 8 days, temperatures of 25 to 35 C,

and culture depths of 7.5 to 20 cm when the monthly average of the daily rate of solar radia-tion was at least 2,000 kcal/m2 day.Experiments conducted during the months of October through December showed that a

culture temperature of 35 C, a shallow culture depth of 10 cm, and a retention time of 8 daysdo not compensate for the loss in algal growth due to the short photoperiod of less than 11h/day and a monthly average of the daily rate of solar radiation of less than 2,000 kcal/m2day. Under these conditions, a retention time of 8 days exceeds the generation time ofCh/ore//a vulgaris 211/8K, so that the harvesting process depletes the algal population andbacterial growth dominates.

When the cell density of Ch/ore//a could be maintained above 10 7 /ml, the culture fluidhad a deep green color which was used as visual evidence of algal growth. The typical odor ofswine manure was absent. The photosynthetic activity of the algae caused daily changes in thepH and the dissolved oxygen content of the swine waste substrate. The magnitude of thesechanges was a function of the intensity of solar radiation, culture depth, and temperature. Ingeneral, the photosynthetic activity of the algae, as measured by the DO content of theculture, increased soon after sunrise and reached a maximum value sometime between 13:00and 16:00 h. The DO concentration frequently rose above 20 mg/I and the pH rose as high as9 in the 20 cm deep cultures and above 10 in the 7.5 cm deep basins. Nighttime respiratoryactivity of algae and bacteria increased the concentration of dissolved CO2 and reduced the pHto about 8 by early morning. During the same time the DO decreased to concentrationsbetween 4 and 6 mg/I. Periods of prolonged cloudiness reduced the photosynthetic activity ofthe algae so that the pH fluctuated only between 7.8 and 8.5 and the DO content between 4and 8 mg/I. Temperatures of less than 25 C also reduced photosynthetically induced changes inpH and DO concentrations.

Cultures with cell densities of less than 10 6 /ml showed no visual evidence of algal growth.The medium had the appearance of activated sewage and the odor was that typical of swinemanure. The pH in these cultures remained at 7.6 and the DO concentrations at less than 1mg/I. This condition was favored at radiation intensities of less than 2,000 kcal/m 2 day incombination with short retention times of 2 to 3 days and culture depths greater than 10 cm.It was concluded that the most effective way of maintaining a high density of Ch/ore//a cellsduring prolonged periods of low intensity of solar radiation was to lower the culture depth to10 cm or less, to stop harvesting, and to stop the addition of fresh substrate to the culture.

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246Contribution of Algal Cells to Total Bioma ss

Efforts were made to distinguish between the contribution made by algal cells and thecontribution made by bacterial cells to the total biomass. Algal cell counts were made using ahemocytometer and Coulter counter. Concentrations of algal dry matter were estimated asthe product of mean cell volume and algal cell counts, taking into consideration a moisturecontent of 75 percent w/w and a specific gravity of the algae of 1.1 g/cm 3 . The mean cellvolume was determined by a MCV-computer attached to the Coulter counter. It was assumedthat the difference between the concentrations of total dry matter obtained by the Milliporefilter technique and the estimated concentrations of algal dry matter represented the contribu-tion made by the bacterial cells to the total biomass.

In sufficiently diluted waste containing 150 to 250 mg NH4-N/I, the highest concentra-tion of algal dry matter, namely 0.70 g/I, was obtained in 10 cm deep basins when the reten-tion time was 6 days, the temperature of the culture was 35 C, and the mean of the intensityof solar radiation was 3,000 kcal/m 2 day. Essentially all of the biomass consisted of algaeunder these conditions. In contrast, the concentration of algal dry matter was 0.36 g/I in the20 cm deep basins and the algae were estimated to have contributed about 50 percent to theconcentration of total biomass of 0.74 g/I.

The total biomass harvested per day from either 10 cm or 20 cm deep basins would thusbe the same in a large scale operation. But the product recovered from the 10 cm deep basinswould be algae whereas the product recovered from the 20 cm deep basins would consist of anequal mixture of algae and bacteria. Given a constant volume of waste which must be treatedeach day, the recovery of an exclusively algal product from 10 cm deep basins would requiretwice as much surface or land area than the recovery of a mixture of algae and bacteria from20 cm deep basins.

At either depth, the concentration of 1,600 mg COD/I of waste water was reduced by 80percent, the concentration of 900 mg BOD/I by 90 percent, and 30 percent of the inorganicnitrogen in the waste was recovered a s organic nitrogen in the biomass.

When the retention time was 4 days instead of 6 days, the concentration of algal drymatter was reduced from 0.70 g/I to 0.45 g/I in the 10 cm deep basins and the algae contrib-uted between 40 and 70 percent of the dry matter to the total biomass. The concentration ofalgal dry matter was reduced from 0.36 g/I to 0.22 g/I in the 20 cm deep basins and the algaecontributed between 20 and 40 percent of the dry matter to the total biomass.

Yields of Algal BiomassMaximum yields of 10 to 12 g/m 2 day of algal dry matter were obtained in 10, 15, and

20 cm deep basins when the retention time was 4 to 6 days, the temperature of the cultures 30to 35 C, and the mean rate of solar radiation 3,000 kcal/m 2 day. Under these conditions,Chlorella vulgaris 211/8K converted 1 to 2 percent of the total radiation into chemical energy.

Although the yields in g/m 2 were similar in the 10, 15, and 20 cm deep basins at both the4-day and the 6-day retention times, operation of the basins at a depth of 10 cm and a reten-tion time of 6 days would be preferred, because the concentration of algal dry matter and the

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247percentage of contribution by the algae to the total biomass were higher for this combinationof parameters than for the other combinations of culture depth and retention time.

When the quantity of algal dry matter recovered from the waste was related to the liveweight of the animals which produced the waste, maximum yields of 2 to 3 g/kg pig per daywere obtained in 7.5 and 10 cm deep basins operated at 6-day and 8-day retention times.

Recovery of NitrogenThe nitrogen balance in the outdoor basins showed that 21 to 47 percent of the ammo-

nium nitrogen available to the algae was converted into harvested biomass. The biomass con-tained 6 to 11 percent nitrogen on a dry matter basis.

The low percentage of conversion of ammonium nitrogen into organic nitrogen wasundoubtedly due in part to the need of the algae to make continuous physiological adjust-ments in response to the daily variations in the intensity of the solar radiation and the pH andDO content of the medium. High concentration of oxygen and pH values above 8 favor photo-respiration, a process which reduces biomass production by inhibiting protein synthesis. Highconcentrations of OH- ions also cause the precipitation of micro- and macronutrients such asiron, magnesium, manganese, and calcium which are essential for photosynthesis and proteinsynthesis. It is therefore essential to control the pH, preferably in the range between 7.5 and7.9, to improve the recovery of nitrogen from swine waste. This range of pH enhances theactivity of the enzymes responsible for the fixation of CO2, in particular ribulose-1,5-diphos-phate carboxylase.

Control of the pH is essential for another reason: 50 to 75 percent of the ammoniumnitrogen in the waste could not be accounted for in the biomass and in the effluent of thebasins. This loss in N was in part attributed to volatilization as NH3 at pH values above 8,induced by the photosynthetic activity of the algae. The remainder of the N not accountedfor settled to the bottom of the basins as sludge.

The bottom sediment or sludge is considered a desirable source of nitrogen and othernutrients when algae are grown in dilute waste such as domestic sewage. In contrast, swinemanure is a rich source of nitrogen so that the layer of sludge no longer serves a useful pur-pose. In fact, the sludge was found to reduce the efficiency of heat transfer from the heatexchanger pipes placed near the bottom of the basins to the culture medium and, more impor-tantly, it served as an ideal environment for the proliferation of algal predators such as Vorti-cella spp. Furthermore, the sludge not only represented a loss of harvestable biomass but alsoan inefficient use of energy because its organic nitrogen had to be recycled again, first intoinorganic nitrogen by bacterial ammonification and then back into organic nitrogen by algaland bacterial assimilation.

Quality of Treated Waste WaterThe water quality of the liquid swine waste was measured in terms of chemical and

biological oxygen demands. Rapid removal of COD and BOD occurred at retention times of2 to 4 days. Thereafter, further increases in the retention time had only a small effect on thereduction of COD and BOD.

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248The COD of the waste pumped into the basins ranged from 700 to 4,800 mg/I and the

BOD ranged from 340 to 2,300 mg/I. Algal-bacterial growth reduced the COD by 80 percent,leaving a COD of 140 to 960 mg/I in the effluent after removal of the biomass by centrifuga-tion. The BOD was reduced by 90 percent, leaving 34 to 230 mg/I in the effluent. Theselevels of COD and BOD are acceptable in a system where the water is recycled for flushing.The residual COD and BOD is too high to permit discharge of the centrifuged waste water intopublic waters.

Stability of Algal CulturesContinuous mixing, at least during daylight hours, was found to be essential for maintain-

ing a predominant and competitive Chlorella population. Chlorella vulgaris 211/8K was ableto dominate the cultures to the practical exclusion of other algal species. Only small numbersof Euglena, Ankistrodesmus, Oscrnatoria, and Scenedesmus spp. as well as diatoms, in particu-lar Nitzchia sp., were observed.

However, an ever present threat to the dominance of the non-motile Chlorella were ce llsof Chlamydomonas spp. in both the motile and palmaloid stages. It was not established whythe number of Chlamydomonas did not increase as long as mixing continued. The paddlewheels turned at 5 rpm, imparting the culture fluid a speed of about 15 cm/sec. When themixing action was stopped, Chlamydomonas cells usually covered the entire basin surface in athick layer of scum within two to three days, thus excluding most of the light from theChlorella population below. A return to mixing would simply move the entire layer along thesurface of the cultures. Attempts to remove the Chlamydomonas by skimming were found tobe ineffective. The layer of cells broke up easily and the cells dispersed into the culturemedium only to form another layer a few hours later. Because of the reduced availability oflight and perhaps the production of antagonistic substances by the Chlamydomonas, theChlorella population declined rapidly.

The opportunistic invasion of Chlamydomonas occurred most often during the months ofMay and June. The organism is a common inhabitant of soil. Its prevalence during these twomonths may have been related to the plowing and tilling activities in the farmlands adjacent tothe experimental facility.

Invasion by predatory rotifers was not observed, but protozoa occurred in large numbers.Those in the circulating culture fluid consisted mostly of ciliates and flagellates which werenon-predatory to the algae. In the accumulating bottom sediment, however, Vorticella,Amoeba, and Paramecium spp. were found to feed on Chlorella. Of these three algal preda-tors, Vorticella was found to be most numerous and voracious when in the actively reproduc-ing stage of its life cycle. It was not established why the number of Vorticella did not increasesufficiently to threaten the survival of the Chlorella population.

Another threat to the stability of the algal population was the apparently random inva-sion of a fungus. During July, 1976, when all 12 basins were operated under identical condi-tions, five basins became infested with a fungus which formed star-shaped particles ranging insize from 5 to 20 mm in diameter. These particles tended to form floating patches of aggre-gated clumps. The infection disappeared on its own within 10 days after its onset.

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249A complete loss of the Chlorella populations was experiences at all culture depths when

the basins were operated at a retention time of 6 days and received rates of solar radiation inexcess of 4,000 kcal/m2 day. Autoflocculation was avoided at a retention time of 4 days. Thespontaneous clumping and settling of the algal cells was attributed to the precipitation ofessential nutrient elements and changes in cell surface charges at pH values above 8.5, inducedby the photosynthetic activity of the algae.

The loss of Chlorella vulgaris 211/8K by autoflocculation can be prevented by control-ling the pH of the culture fluid. Control should be exercised between pH 7.5 and 7.9 by CO2injection to keep micro- and macronutrient elements available to the algae in soluble form andto maximize production of the enzyme ribulose-1,5-disphosphate carboxylase in the carbonreduction cycle for maximum photosynthesis. Although total quantities of essential elementssuch as iron, magnesium, manganese, calcium, and sulfur were found to be adequate in theswine manure for algal growth, the availability of these elements to the algae in a metabolicallysuitable form may actually be less than indicated, so that the supplemental addition of a mix-ture of minerals may be advisable.

The loss of cultural stability due to the invasion of predators, fungi, and undesirablespecies of algae remains unpredictable until research determines which factors enable Chlorella,or any other chosen alga, to maintain predominance. Continuous mixing and control of pHare but two such factors. A third factor may be the recycling of a portion of the harvestedalgae to increase the concentration of Chlorella in the basins.

Harvesting of AlgaeThe algal-bacterial biomass in the basin effluents was concentrated to a paste with a solids

content of 15 to 20 percent on a dry matter basis. The centrifuge which was selected for har-vesting the biomass is commonly used in industry to separate crystals and other heavy particlesfrom solutions. It was acquired for the experimental facility because of its low cost of $6,800relative to a cost of $13,000 to $15,000 for the smallest centrifuges available on the marketfor concentrating single cell protein. The centrifuge was adequate for harvesting the algae forfeeding trials but it was found impractical for large scale harvesting. Its disadvantages include afairly large motor of 2.2 kW, a low centrifugal force of 2,000 x G, and the necessity to stopoperations periodically to scrape the algal paste from the centrifuge bowl.

At a flow rate of 5 1/m in, the centrifuge removed about 75 percent of the algal cells whilemost of the bacterial cells passed through because of the low centrifugal force. At a flow rateof 3 l/min, approximately 85 percent of the algae were recovered. Given the highest concen-tration of algal dry matter produced in the outdoor basins, namely 0.70 g/I, and a cost of50/kWh of electricity, the cost of concentrating the algae was $0.69 to $1.02 per kg.

If a centrifuge specifically designed to harvest single cell protein had been available, thecost of concentrating the algae would have been $0.07 per kg algae or $0.13 per kg of crudeprotein. This compares with a cost of $0.50 to $0.70 per kg of soybean meal protein, F.O.B.Portland, Oregon, 1977. To be competitive with soybean meal as a protein supplement inlivestock rations, the cost of producing and processing the algae, in addition to the cost of har-vesting should therefore not exceed $0.40 to $0.60/kg.

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250The most efficient and reliable methods of harvesting micro-algae, but also the most

expensive ones, are centrifugation and chemical flocculation. As compared to centrifugation,chemical flocculation has the disadvantage of contaminating the algal product with the coagu-lating chemical, usually alum. This makes the use of the algae as a feed supplement question-able because of possible toxicity effects as well as reduced digestibility and palatability.

The least expensive but also the least efficient methods of harvesting algae are sedimenta-tion, autoflocculation, and filtration through microstrainers. The efficiency of microstrainerscould be improved significantly by growing filamentous algae instead of single-celled orcolonial algae such as Chlorella and Scenedesmus and by increasing the concentration of thefilamentous algae. Treatment ponds with continuous flow-through of waste water usuallycontain mixed populations of algae. At short retention times, single-celled or colonial typegreen algae predominate over slower growing, filamentous blue-green algae such as Oscilla-toria. By lengthening the retention time and recycling a portion of the harvested algae to thepond it may be feasible to establish the slow growing filamentous algae as the predominantspecies over the fast growing unicellular algae.

The selection of filamentous algae for the purpose of decreasing the cost of harvesting hasseveral disadvantages, however. Filamentous algae are most often blue-green algae, many ofwhich are known to produce potent toxins lethal to both man and animal. Strict operationalcontrols as well as product quality controls must therefore be maintained to ensure the safeuse of the final product as a protein supplement in livestock rations. Furthermore, the longretention time needed to assure the predominance of filamentous algae, as compared to theshort retention time needed to maintain the predominance of unicellular algae such asChlorella, will require a correspondingly larger surface area of the algae pond in order to treatthe same volume of waste water each day.

Thus trade-offs have to be considered in evaluating the cost of harvesting algae. Micro-straining filamentous blue-green algae is less costly than centrifuging unicellular green algaesuch as Chlorella and Scenedesmus, but some blue-green algae are known to produce deadlytoxins whereas green algae are not known to produce toxins and, given a fixed volume of wastewhich needs to be treated each day, the filamentous algae require more surface/land area forgrowth than the unicellular algae because of the difference in growth rates.

Growing Chlorella or some other green algae and harvesting them by centrifugation maytherefore be more cost effective for a large scale operation than growing filamentous algae andconcentrating them by microstrainers.

Nutritional Value of AlgaeThe harvested biomass was freeze-dried and its nutritional value determined in feeding

trials with Long-Evans rats. It was found that the raw algae had a low protein efficiency ratio(PER) of 0.84 which improved to 131 by steam autoclaving for 30 min at 120 C. The digesti-bility of the raw algae was 59 percent which improved to 68 percent after autoclaving. Incomparison, the digestibility of soybean meal is 85 percent.

Although the PER was low, when used as a protein supplement to corn, the algae gavefavorable growth and feed efficiency responses. At a dietary level of 18 percent, the algalprotein performed as well as fish and soybean meal at a dietary level of 13 percent.

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251Amino acid supplementation of a corn diet containing autoclaved algae, indicated that

lysine was the first-limiting amino acid while methionine was present in adequate quantities.A corn-algae diet supplemented with 0.3 percent lysine gave a growth rate of 5.5 g/day,exceeding a growth rate of 5 g/day obtained with a corn-soybean meal diet at the same proteinlevel.

Because the lysine content of the raw algae was high, namely 4.83 percent on a drymatter basis, further study is needed to identify the reasons for its low availability. Lysineinactivation during heat processing and lysine "tie-up" as constituent of the indigestible cellwall are two possibilities to consider. Development of processing methods to increase proteindigestibility and the availability of lysine are needed before the algae can be used successfullyas a replacement for soybean meal in swine rations.

Bacterial GrowthBatch cultures of mixed populations of bacteria indigenous to the liquid phase of swine

manure reached maximum concentrations in 6 h, at which time the cells could be harvested.Temperatures in the range between 15 and 35 C did not influence maximum concentrations ofbacterial dry matter but determined which species of bacteria were present.

Maximum concentrations of bacterial dry matter increased linearly as a function of theconcentration of ammonium N in the range from 100 to 800 mg/I. The crude protein contentwas quite variable and often below 50 percent of the dry matter. This was attributed to theoccurrence of autolysis. Maximum concentrations of bacteria in batch cultures are obtainednear the stationary phase of growth when many species of bacteria undergo autolysis.

The recovery of nutrients from swine waste by bacteria would be more efficient in acontinuous culture system which maintains the bacterial populations at an exponential rate ofgrowth. Autolysis would be largely avoided and a crude protein content of 50 to 75 percentof the dry matter could be expected. Yields of 0.38 g cells/g COD and yields of 2.2 g cells/kgpig per day appear feasible.

The continuous culture of bacteria is probably the simplest biological method of recover-ing nutrients from swine waste. Many of the different genera and species of bacteria indigen-ous to the swine waste tolerate or can adapt to a wide range of pH, temperature, and nutrientconcentrations. The need to control particular environmental and nutritional conditions istherefore reduced to a minimum and the system can be operated throughout the year. Auto-mated systems for the continuous culture of bacteria are commercially available.

However, given the genera and species of bacteria present in swine waste, it is conceivablethat toxic substances and organisms which are potentially pathogenic to both man and animalcan be produced. The harvested biomass may therefore have to be processed to eliminate thepotential problems of toxicity and pathogenicity before the bacteria can be used as a proteinsupplement in livestock feed. This aspect of nutrient recovery by bacteria from swine wasteneeds support for further research.

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252Anaerobic Digestion

Operational CharacteristicsThe settled manure solids from 50 pigs were loaded into an underground, concrete

digester with a volume of 14 m3 at a rate of 0.96 kg VS/m3 day (0.06 lb VS/ft3 day). Thedigester content was continuously mixed and maintained at a temperature of 37 C. Its pHstabilized at 7.2. The hydraulic residence time was 47 days. The digester influent had aCOD/VS ratio of 1.5 and a VS/TS ratio of 0.8.

Gas ProductionThe digestion process removed 55 percent of the total solids from the swine manure.

The destruction of the volatile solids averaged 56 percent, and the COD was reduced by 41percent. These values are in general agreement with values cited in the literature for smalllaboratory digesters ranging in size from a few liters to several hundred liters.

The daily gas production averaged 8.4 m3 (300 ft3 ), containing 68 percent CH4 and 32percent CO2. Based on the destruction of volatile solids, the production of biogas corres-ponded to 1.06 m3 /kg VS removed (17.2 ft3 /Ib VS). This value is consistent with valuesobtained for laboratory sized digesters.

Energy RequirementsThe energy requirements for heating the digester ranged from 13,000 kcal/day during the

summer to 64,000 kcal/day during the winter. These values are greatly exaggerated because ofthe unusual arrangement of placing two concrete tanks side by side into the ground and con-necting them to make a single functional unit, and because the top surfaces of the tanks wereused as work areas and left exposed to the air without insulation.

Potential for Improving Yields of BiogasThe production of biogas may be enhanced by the following methods:1. The loading rate may be increased from 0.96 kg VS/m 3 day to 3.8 kg VS/m3 daywhile decreasing the hydraulic residence time from 47 days to 10 to 17 days.2 . The production of biogas with a high fuel value is favored when the C/N ratio is near30. Swine manure has a C/N ratio of approximately 10. By adding properly pretreated cellu-

losic wastes to the digester, such as grass and cereal straws as well as other crop residues, theC/N ratio of the swine manure may be raised to 30 and the yield of CH 4 by 0.44 m3 /kg ofcellulose.

Pure cellulose is readily digested under anaerobic conditions. However, in its natural stateit is chemically bound to hemicellulose and lignin in a complex structure which is largelyinaccessible to the extracellular enzymes of the bacteria in the digester. Pretreatment of thecellulosic waste is therefore necessary.

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253The practical feasibility of pretreating cellulosic wastes and adding them to manure to

improve the yield of biogas remains to be determined.3 . The growth of yeast or microfungi in the liquid phase of diluted manure would gen-

erate considerable quantities of heat which could be diverted to the anaerobic digester. Bycoupling the heat producing aerobic fermentation of yeast or microfungi in the liquid phase ofthe manure with the heat requiring anaerobic digestion of the manure solids, the energyrequirements normally expected for cooling of the yeast fermenter can be eliminated while atthe same time the energy requirements of the digester for heating can be reduced, if not elimi-nated also. Most of the biogas could then be used for purposes other than heating the digester.

4. At present, the anaerobic digestion of organic wastes is practiced most often in asingle vessel. However, the digestion process itself is mediated by two distinct populations ofbacteria which differ from each other in growth characteristics and sensitivity to environmen-tal stress.

Separating the acidogenic and methanogenic populations of bacteria and permitting themto grow in separate reactors under environmental and nutritional conditions most suitable foreach group, may optimize the anaerobic digestion of organic matter. Phase separation may beexpected to provide substantial benefits, including greater removal of volatile solids and there-fore higher yields of biogas as well as reduced digester volume and capital cost requirements.

ProblemsAn important element in the development of the anaerobic digester for farm use is the

acceptance of the technology by the farmer. The probability of acceptance is increased if thedigester system returns a profit, which may not necessarily be a monetary one, if the digestersystem is easy to understand, install, and operate, and if adequate service is available.

The quantity and quality of manure available for digestion from a livestock operation orfarming enterprise are likely to vary during the year. The energy needs also vary from seasonto season. An important research need is therefore the development of systems of wastemanagement which aim at the full utilization of organic wastes from all sources, includingmanure and crop residues, and find a continuous use for the gas.

Energy Balance of Basins

Solar radiation, atmospheric radiation, back radiation, convection, evaporation, precipi-tation, and the addition of cold water to replace losses due to harvesting and evaporation con-tributed to the energy balance of the basins.

Equations for each component of heat transfer were developed and programmed into acomputer to predict the net energy requirements of outdoor basins under varying environmen-tal conditions.

Energy requirements measured for outdoor basins at 30 C ranged from 27.8 mcal/cm2secin February to an average of 10.6 mcal/cm 2 sec in August. Energy requirements for culturesmaintained at 15 C ranged from 9.8 mcal/cm2 sec in February to 4.8 mcal/cm2 sec in April.

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254The computer model predicted the energy requirements within 10 percent of the meas-

ured values. The model indicated that 73 to 115 percent of the net energy loss was due toevaporation and convection for the 30 C cultures. The relative contribution made by radiationto the total loss of energy varied from a net energy gain of 20.3 percent in July to a net energyloss of 17 percent in January. An average of 5.6 percent of the loss in energy was due to theaddition of make-up water.

Methods of Nutrient and Energy RecoveryThe necessity to dilute the swine waste with large volumes of water in order to make the

liquid phase of the manure suitable for algal growth, and the potentially high cost of harvest-ing and processing the algae prompted further considerations of the biological recovery ofnutrients and energy from swine manure.

To reduce the large volumes of dilution water, it is proposed to pretreat the liquid wasteby first using it as a substrate for the growth of selected heterotrophs such as yeasts and micro-fungi. As an alternative to harvesting the algae by centrifugation and to avoid the potentiallyhigh cost of processing the algae to improve their low digestibility and the low availability oflysine, it is proposed to use a polyculture of fish, thereby gaining a high quality protein whichis easy to harvest and already accepted as a supplement in livestock feed.

An effort was made to bring the several possible bioconversion processes together intofunctional management systems, which have in common that the solids are treated by anaero-bic digestion to recover biogas, to solubilize nutrients, and to reduce the solid waste by 40 to60 percent. Most also conserve water by recycling the waste water to flush the animal quartersand dilute the manure. The harvested protein is recycled as a supplement in livestock feed.

The proposed bioconversion methods include:OPTIONA: Methane and Fertilizer

The manure is digested to produce gas and a biologically stabilized waste for use as afertilizer.OPTION B: Methane and Algae

The manure is digested to produce gas and to solubilize plant nutrients. The nutrients inthe liquid phase of the digested manure are recovered by algae.OPTION C: Methane, Yeast, Microfungi, and Algae

The manure solids are digested to produce gas. Yeast, microfungi, and algae are culti-vated in the liquid phase of the manure.OPTION D: Methane and Fish

The manure is digested to produce gas. The digester effluent is used as a source of nutri-ents for fish.

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OPTION E: Methane and Bacteria

The manure is digested to produce gas from the solids. The liquid portion of the manureand the supernatant liquid from the digester are treated in aeration basins, operated to maxi-mize the production of bacteria.

Architectural perspectives and plan views were developed together with schematic dia-grams showing the flow of energy and materials through each system based on the feed andenergy needs and the waste discharge of 100 pigs.

The quantities of biogas and protein which can be recovered from swine waste are limitedby the availability of carbon and nitrogen. Algae, yeast, or microfungi could recover about 5percent of the digestible energy present in the feed or replace 30 to 60 percent of the soybeanmeal protein. About 90 percent of the combustible energy present in the manure could berecovered. However, yields of each bioconversion process can be increased as desired bysupplying the bioconversion units with additional sources of carbon and nitrogen. Technicaldifficulties need to be overcome, principally the conversion of cellulosic wastes to reducingsugars, the stability of the algal cultures under field conditions, the efficient transfer of oxygento growing cells of yeast, bacteria, or microfungi, and improvement of the anaerobic digestionprocess by phase separation. No new technologies need be developed.

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