Cheese whey anaerobic digestion — Effect of chemical flocculant addition

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<ul><li><p>orechnology Letters Vol 5 No 3 153-158 (1983) </p><p>CHEESE WHEY ANAEROBIC DIGESTION - EFFECT </p><p>OF CHEMICAL FLOCCULANT ADDITION </p><p>I.J. Callander* and J.P. Barford </p><p>Department of Chemical Engineering The University of Sydney, N.S.W. 2006 Australia </p><p>SUMMARY </p><p>Two stirred tank digesters were operated with biomass retention by internal settling during effluent removal. In one digester a flocculating agent was used to enhance microbial aggregation and settling. The unflocculated digester achieved a maximum stable loading rate of 8-8.5kg COD/ma. day and biomass density of 19.39 VSS/l compared to the flocculated digester which achieved a (non-maximal) stable loading rate of 12.3kg COD/m3. day and biomass density of 43.39 VSS/l. than 97%. </p><p>Both digesters had a COD conversion efficiency greater Operation of a stirred tank digester in a mix/settle mode </p><p>allows a significant increase in biomass levels over conventional continuously stirred digesters. The addition of a chemical flocculant significantly enhances this improvement. </p><p>INTRODUCTION </p><p>Cheese whey, essentially a 5% lactose solution, is the principal wastewater from cheese factories. The importance of its utilisation or disposal is indicated by the large range of techniques investigated, including physical, chemical and biological processes. Anaerobic digestion is a biological process suitable for the disposal of cheese whey. The potential to produce biogas which may be used for raising steam in the cheese factory together with low sludge production makes anaerobic digestion an attractive option for many cheese making facilities. Most whey digestion work to date has employed mechanically agitated digesters with or without recycle (Holder and Sewards, 1976, 1981, Harischandra and Saxena, 1969, Follman and Markl, 1979). Even with biomass recycle, low biomass densities of 2-109 VSS/l were obtained. At loadings up to 6kg BOD/ms. day, BOD conversions were 85-95% (Holder and Sewards, 1976, Parker, Parker and Lyons, 1979). Higher loadings, up to 38kg COD/m3. day, have been sustained with a fluidised bed reactor although at the expense of COD conversion - 70% at the highest loading (Hickey and Owens, 1981). </p><p>The anaerobic digestion research programme at the University of Sydney has been based on the development of an up-flow biochemical reactor and stirred tank digesters operating in a mix/settle mode utilising natural flocculation or induced flocculation (using synthetic organic flocculating agents) (Callander, 1982). The main features of this development are the achievement of enhanced productivity and process efficiency (leading to reduced capital and operating costs) by the achievement and retention in the digester of much higher viable biomass </p><p>*Present address: Biotechnology Section, Forest Research Institute, Rotorua, New Zealand </p><p>153 </p></li><li><p>levels than can be achieved in simple batch and/or continuous stirred tank arrangements. </p><p>This work describes the operation and comparison of performances of two stirred tank digesters operated identically in a mix/settle mode, with the exception of one using a flocculating agent to enhance microbial aggregation and settling. The incremental benefit of chemical flocculant addition is assessed. </p><p>MATERIALS AND METHODS </p><p>Quickfit 2 litre flasks were used with standard fittings and a working volume of 1800 mls (Figure 1). These were operated in a mix/settle mode as follows. From day 27, feeding and effluent removal were automatic using a cam timer with a three hour cycle. Digester feeding and supernatant removal occurred following biomass settling (Figure 2). Previously, the digesters were fed manually, once daily at the same time. The mixing was stopped, thecontents allowed to settle for lo-20 minutes to provide a supernatant relatively free of biomass floes and a calculated volume of supernatant removed via a sample port using a vacuum apparatus. The same volume of refrigerated feed solution was then added to the digester. </p><p>Temperature was controlled at 35C (+O.l"C). Stirrer speed was set to 50 *lOrpm. This speed was just sufficient for complete mixing yet was low enough to minimise floe rupture due to turbulent shear forces. </p><p>The whey solution used was a non-hygroscopic cheese whey powder dissolved in a nutrient solution. Whey powder was obtained from Ibis Milk Products Ltd, Shepparton, Victoria. The manufacturer supplied the following approximate analysis (all weight X): moisture 2.5-4.0; fat 0.8-1.5; protein 11.0-14.0; nitrogen 1.8-2.2; NaCl 2.5-4.5; phosphorus 0.75-0.85; P20 potassium 2.1-2.5; fibre ; lactose 72. As whey is typically 6.6% 8 </p><p>1.6-2.0; calcium 0.65-0.75; sodium 0.65-0.75; </p><p>solids, 73.019 of this powder was added to 1 litre of nutrient solution. The need for a nutrient supplement was determined from a previously published assessment of nutrient requirements in anaerobic digestion (Speece and McCarty, 1964). This consisted of (all mg/l): (NH4)2HP04 707; MgS04.7H20 251; MgC12.6H20 1,312; KC1 491; FeC13.6H20 2,048. When freshly made up the solution had a pH of 4.7. Throughout the experiment the whey pH was raised in steps to 6.5 with sodium hydroxide in order to control digester pH between 6.8 and 7.2. To minimise microbial breakdown of the feed during storage, the feed containers were washed and replenished every l-2 days. </p><p>Each digester was seeded with 1.8 litres of digested sludge taken from the anaerobic digester circulation line at Bondi Sewage Works, Sydney. The sludge was first strained through cheesecloth to remove coarse solids, taking care to minimise contact with air. After transfer to the digesters, the head space was purged with lo-20 volumes of nitrogen. </p><p>An initial space loading was chosen to give a biological loading of approximately O.lkg COD/kg VSS. day (Lettinga et al,. 1980) to ensure a stable process start up. Space loadings were increased in small steps of about 0.5kg COD/m3. day when low digester fatty acid levels (less than lOOmg/l acetic, 20mg/l propionic) indicated balanced activities of non-methanogenic and methanogenic organisms. </p><p>The following parameters were routinely monitored: individual volatile </p><p>154 </p></li><li><p>Feed Elflunt </p><p>Product Oar </p><p>f </p><p>P </p><p>mt ear *tar </p><p>P Qar sampb </p><p>Point </p><p>Figure 1 Whey digester </p><p>r-x- 10 7- mins 20 mins Faad Mixar Digastw Mixar Faad/Effluant </p><p>Pump l+ ML-- </p><p>1 min. I </p><p>L------ 2.1 min. 3 hours -1 </p><p>Kay m On </p><p>0 Off </p><p>Figure 2 Whey digester - cam timer cycle </p><p>155 </p></li><li><p>fatty acids, pH, gas composition, volatile suspended solids of digester contents and effluent, COD balances and nitrogen and phosphorus balances. A wide range of commercially available synthetic flocculating agents were tested on the basis of their effectiveness in biomass settling, lack of toxicity and non-degradability (Callander, 1982). Zetag 88N (Allied Colloids, Sydney, Australia) was selected. Dosage frequency and quantity were determined by biomass levels in the digester effluent (Callander, 1982). </p><p>RESULTS AND DISCUSSION </p><p>Figures 3 and 4 demonstrate the performance of the two digesters. The experimental results may be divided into three sections, phase 1 (day l-76), phase 2 (day 77-147) and phase 3 (day 148-204). </p><p>During phase 1 (day l-76), no flocculant addition occurred. This provided a demonstration that both digesters had acclimatized to the whey (low volatile fatty acids and increased loading rates) and that there were no significant differences in digester construction or operation which could invalidate the conclusions drawn about the use of flocculant during phases 2 and 3 of the experiment. By the end of this phase, treatment efficiencies (98.4% COD reduction) were excellent (Table 1). </p><p>Flocculant addition commenced on day 77 with both digesters stable (low volatile fatty acids) and biomass density and loading rates essentially equivalent (approx. 18.59 VSS/l and 3kg COD/m3. day respectively). During phase 2 (day 77-147), the difference in digester performance attributable to the use of a flocculating agent and the maximum loading of the digester without flocculant addition are clearly demonstrated. Figure 3 shows that from day 69 to day 119, the biomass density in the unflocculated digester remained essentially unchanged (19.3 to 19.19 VSS/l) while that of the flocculated digester rose from 17.4 to 28.39 VSS/l, a 63% increase. By day 131, the flocculated digester had risen to 29.5g VSS/l while on day 147 the unflocculated digester had decreased to 17.19 VSS/l, a 73% difference.. Again treatment efficiencies were excellent, 97.1% COD reduction for the unflocculated digester and 99.2% for the flocculated digester (at day 110) (Table 1). Volatile fatty acid levels were low in both digesters until the unflocculated digester began to fail on day 140. This was characterised by rising volatile fatty,acid levels, falling conversion efficiencies, reduced gas production and falling pH. Without flocculant addition, stable, efficient digestion could not be maintained in excess of 8-8.5kg COD/m3. day. </p><p>During phase 3 (day 148-204) an improvement in digester performance effected by the use of flocculating agent is illustrated. This period does not describe the maximum potential for improvement using flocculants. This is the subject of a more extensive publication (Barford et al, 1983). During phase 3 (day 148-204) the biomass level had increased to 43.39 VSS/l by day 201 and the loading rate to 12.3kg COD/m3. day. These represent increases of 153% in biomass levels and 44.7% in loading rates over the maximum performance of the unflocculated digester. Again treatment efficiencies were excellent (&gt;99% COD reduction) (Table 1). Volatile fatty acid levels were low indicating good digester stability. </p><p>The quantities and timing of flocculant addition have been described elsewhere (Callander, 1982). Total flocculant cost was small when </p><p>156 </p></li><li><p>DAY </p><p>:i 68 </p><p>110 152 1Yb </p><p>se- </p><p>25 - </p><p>28 - </p><p>IS- </p><p>18 - </p><p>s- </p><p>e I I I I 0 60 188 Isa 200 </p><p>t IMCdoyd </p><p>Figure 3 Whey digesters - Biomass density comparison. </p><p>14 </p><p>2 </p><p>k % </p><p>,2 </p><p>0 0 188 </p><p>tlnrCdoyr&gt; </p><p>Figure 4 Whey digesters - Loading rate comparison </p><p>FEED COD w/l </p><p>63,200 66,400 69,600 68,700 64,100 69,800 </p><p>Table 1 - Feed COD and COD Conversion </p><p>% COD REDUCTION % COD REDUCTION (TOTAL) (suPERNATANT) </p><p>unflocculated flocculated unflocculated flocculated </p><p>92.0 91.3 </p><p>98.4 98.4 73.4 98.0 97.1 99.2 15.6* 89.7 37.8 99.4 </p><p>97.9 99.4 </p><p>*Unflocculated digester had failed </p><p>157 </p></li><li>compared to the value of the methane generated (~4% based on fuel oil prices). No attempt was made in this work to optimise flocculant addition with respect to the duration and extent of digester mixing. This is discussed elsewhere (Barford et al, 1983). Throughout this experiment gas composition was steady (51-56%. CH4) and volatile fatty acids generally low (</li></ul>

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