Anaerobic Co-Digestion of a Single Source Oily Waste and High Strength Pet Food Wastewater: A Study of Failure and Revival of a Full Scale Digester
This paper focuses on the revival of a formerly failed digester of 1800 m3 volume. The most obvious cause of failure was identified to be due to capping caused by foam and scum, as result of attempting to treat oil rich, high strength wastewater. The revival was affected by implementing co-digestion in a 3-step remedial procedure. Though in the typical sense, codigestion involves separate waste streams, here a single waste stream was manipulated to apply the concept of co-digestion. Through the virtual co-digestion the digester presently succeeds to treat the daily plant effluent flows of 50 m3 with COD > 45 g/L and around 10 tons of sludge/day which has around 20% oil and fat, exhibiting a COD removal > 90%.The digester operates at 35±1 0C and HRT of 30 days with loading rates of 3.4 kg COD/m3d and 1.3 kg O&G/m3d. The biogas generated from this digester is sufficient to operate a 40 hp boiler at 100 psi.
Anaerobic digestion is a particularly attractive treatment solution for high strength wastewaters due to the operational economy and generation of biogas. However, in the absence of close monitoring and operational expertise, large numbers of anaerobic systems have failed across North America. Industrial wastes are generally deficient in methanogens. Hence, industrial digesters require to be seeded with methanogen- rich sludge such as primary sludges from municipal digesters or fresh cow manure. Above this, anaerobic processes’ susceptibility to high oil and grease has been widely recognized .
Animal oil and grease are highly organic and therefore an excellent source to generate biogas. But their hydrophobic nature limits their availability for biodegradation, and results in floating and foaming in both aerobic and anaerobic systems. To treat oil and grease rich wastewaters biologically the most important criteria to be met are stable and active biomass and proper mixing as emphasized by Hashimoto . In spite of the numerous reports of failed anaerobic treatment systems in the presence of oil and grease, Fermandez et al.  successfully treated up to 28% animal fats anaerobically by co-digesting it with cow manure. Similarly Kim et al  proved co-digestion of food wastes with sewage sludge to enhance biogas generation and solids destruction. Co-digestion is the process of digesting two separate waste streams in a single bioreactor. In co-digestion, the favorable wastes aid stabilization of the biological system and complex wastes are ‘co-digested’.
The anaerobic digester of 1800 m3 volumetric capacity had failed to treat the pet food plant effluent for 4 years when the concept of co-digestion was employed in the, now, successful attempt to revive this digester. In this study co-digestion was implemented not in the typical style of multiple feed streams but by manipulating the single waste stream. A 3-step remedial procedure was undertaken to implement co-digestion. The first step involved the separation of oil from the waste stream to generate relatively oil-free wastewater. In the second step the digester was acclimatized to the relatively oil-free wastewater stream, and in the third step co-digestion of an oil rich stream was achieved.
The pet food industry effluent treated in this digester is characterized by O&G concentrations as high as 60000 mg/L, total COD, BOD, TKN and ammonia concentrations of 100000 mg/L, 80000 mg/L, 2500 mg/L and 1200 mg/L respectively (Table1). The successful implementation of co-digestion (in a broader interpretation) in this case offers an innovative solution for many failed anaerobic digesters across North America.