wastewater recycling in anaerobic digestion of beef cattle wastes
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Agricultural Wastes 7 (1983) 1-12
Wastewater Recycling in Anaerobic Digestion of Beef Cattle Wastes
A. C. Chang, W. C. Fairbank, T. E. Jones & J. E. Warneke
Department of Soil and Environmental Sciences, University of California, Riverside, CA 92521, USA
A BSTRA C T
Six bench top mesophilic anaerobic digesters were used to determine the feasibility of recirculating the digester's wastewater effluent during the anaerobic fermentation of beef cattle wastes. The results indicated that wastewater recycling in anaerobic fermentation of beef cattle wastes increased the pH and salinity levels of the digester contents. At high digester loading (> 11.6g Total Solids per litre of digester volume per day) and high rates of wastewater recycling (> 80 %), there was also significant reduction in the percentage of volatile solid destruction and in gas generation.
The production of methane gas (e.g. biogas) through anaerobic fermentation of selected biomass and organic wastes has attracted widespread attention in recent years. In addition to the benefit of energy recovery, it was believed that residues from anaerobic fermentation of orgarAc wastes could also yield a high-protein livestock feed ingredient (Meckert, 1978; Hashimoto et al., 1978).
Since 1978, a pilot plant utilising wastes from a beef cattle feedlot to produce biogas has been operating near Brawley, California. The project was jointly funded by Pacific Gas and Electric Co. (San Francisco, California) and Southern California Gas Co. (Los Angeles, California).
Agricultural Wastes 0141-4607/83/$03"00 Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
2 A.C. Chang, W. C. Fairbank, T. E. Jones, J. E. Warneke
The 67.4M 3 (active volume) experimental digester received 4-3 M 3 of digester feed daily consisting of 2361 kg beef cattle manure (approxi- mately 7 days old, collected at a nearby feedlot) and 3045 kg water. The facility has the capacity of handling wastes from 200 head of cattle (weighing from 300-460 kg). The operating parameters and performance measurements of the pilot digester are summarised in Table 1.
TABLE 1 Operating Parameters and Performance Characteristics of the Pilot Digester
Active volume Hydraulic retention time Operating temperature Solid reductions
Total Solids Volatile Solids
Digester gas Volume Composition (by volume)
67.4M 3 20 days 37"8 C
42.5-51.0 M3/day CH 4 58/o, CO 2 38~o, other 4~o
The digester liquor withdrawn each day was centrifuged to extract the solid fraction. The recovered fibre solids were sun dried, then blended with other feed ingredients. The centrate, which has high pollution potential, requires proper disposal (Table 2). In a previous investigation, the constraints and alternatives for disposing of this wastewater in the Imperial Valley were analysed (Chang & Fairbank, 1981). It was concluded that wastewater disposal would be the greatest limiting factor in scaling up the pilot digester to a full operation. All of the wastewater treatment alternatives considered (e.g. joint treatment with domestic wastes, land disposal, sand bed drying and evaporation ponds) had serious limitations.
Water was added daily to liquefy the manure, so it was logical to consider using the centrate in preparing the digester feed slurry. If recycling of wastewater back into the digester proved successful, it could eliminate the need for wastewater disposal. Even a partial recycling should reduce the amount of effluent requiring final disposal. However, reintroducing the centrate to the digester might affect the performance of the anaerobic fermentation process. This paper summarises the results of a bench top experiment set up to examine the effects of effluent recycling on the anaerobic digestion of beef cattle wastes.
The microbiology and biochemistry of anaerobic degradation of
Wastewater recycling of beef cattle wastes
TABLE 2 Characteristics of Digester Feed and Wastewater Effluent (Centrate)
Generated by the Pilot Digester
Parameter Digester feed Effluent
pH 6"0-6-5 8.0-8"6 Electrical conductivity (mmho cm- 1) 11.5 11"5 Total Solids (~o) 5.0 3.3-4.1
Volatile Solids ( ~o TS) 66 58 Fixed Solids ( ~o TS) 34 42
Biochemical Oxygen Demand (mg litre- 1) _ 1 600-2 200 Chemical Oxygen Demand (mg litre- 1) - - 31 000-45 000 Total Organic Carbon (mg litre- 1) 6 000
organic wastes have been studied in recent years (Kirsch & Sykes, 1971; Hobson et al., 1974). Based on the microbial kinetics, anaerobic fermentation with feedback of the reactor effluent should help to overcome the slow growth rate of anaerobic bacteria and to increase the efficiency of biochemical conversion. Anaerobic fermentation reactors based on this concept have been mathematically analysed and experimentally demonstrated (Herbert, 1961; Fencl, 1966; Pirt & Kurowski, 1970). There have also been examples of successful recycle in full-scale treatment systems (Torpey & Melbinger, 1967; Schroepfer et al., 1955). However, the purpose and nature of recycling the effluent in this case was entirely different. Biogas production using beef cattle manure produces wastewater which gives a difficult disposal problem. If effluent can be used for make-up water without interfering with the performance of the anaerobic fermentation process, the wastewater volume can be greatly reduced.
The experiment was initiated by reproducing the operating conditions of the pilot-plant digester (Table 1) in six bench top anaerobic digesters. The anaerobic fermentation reactors were made from polyethylene tubing, 14 cm inside diameter and 38 cm in height. They were partially submerged in a water bath to maintain 35 C _+ 1 . Each reactor was filled with 2 litres of digester contents obtained from the pilot digester and then fed daily with the feed prepared for the pilot digester until its performance stabilised (approximately 15 days). Digesters were slow mixed for 15 min
Wastewater recycling of beef cattle wastes 5
every 2 h. The mixing actions were synchronised by fastening each centre shaft on to a jar test apparatus drive.
Once the digesters were normalised to the laboratory environment, each was converted to operate at an effluent recycling condition of 0, 20, 40, 60, 80 or 100 % daily. Based on the data collected in the field, as much as 75 ~o of daily water input would be recovered as wastewater effluent during the centrifugation. At 100% recycling, the entire amounts of wastewater were re-turned into the digester. In the laboratory, the effluents for recycling were simulated by centrifuging the daily draw-off of each digester at 1500 rpm for 15 min. The daily digester feed was prepared by mixing the waste solids with given amounts ofwastewater. The remaining daily water requirement of the digester feed was made up from tap water.
The digesters underwent four loading stages with gradually increased feeding at each stage. The Total Solids and Volatile Solids inputs for each loading stage are summarised in Table 3. To maintain the hydraulic loading time at 20 days, the Total Solids input at each loading stage was increased but the volume of the digester feed remained unchanged. For a digester without any effluent recycling, the Total Solids input rose from 7.10g litre -1 of digester volume per day at stage I to 15.70gday -1 at stage IV. There were additional increments in Total Solids and Volatile Solids inputs due to effluent recycling. The digesters were allowed to equilibrate with the current input loading (greater than 20 days) before any performance data were collected.
Electrical conductivity, pH and gas volume (expressed as volume at 1 atmosphere and 0 C)were recorded daily. The composition ofbiogas was analysed once a week using gas chromatography (Model 25V Fisher Gas Partitioner, Fis