Slaughterhouses produce high strength wastewater (EC, 2005), which contain high levels of biodegradable organic matter, as faecal, undigested food, blood, suspended material (Jian and Zhang, 1999). Slaughterhouse wastewater composition in terms of organic strength, inorganic elements, alkalinity, and pH is adequate for biological treatment (Massé and Masse, 2000). Design criteria for slaughterhouse wastewater treatment plants are widely published (Travers & Lovett, 1984; Li et al, 2008).
Generally, variations in slaughterhouse wastewater composition are significant, depending on the production procedures, byproducts’ recovery and cleaning procedures (Pozo et al., 2003). The high biochemical oxygen demand (BOD) to chemical oxygen demand (COD) ratio suggests an easily biodegradable wastewater, the relatively low BOD to total Kjeldhal nitrogen (TKN) ratio suggests a need for nitriﬁcation and possibly even nitrogen removal and finally the relatively high total suspended solids (TSS) characterize the slaughterhouses’ wastewater (Chen and Lo, 2003; Lovett et al., 1984). The TKN values are highly dependent on the blood handling process of the slaughterhouse. The concentration of contaminants in slaughterhouses wastewater vary, with COD, TKN and TSS concentration ranging from 1000-20000 mg L-1, 150-10000mg L-1 and 250-5000mg L-1 respectively (Li et al, 2008). For large scale slaughterhouses, on-site biological treatment is recommended by the European Commission to remove organic carbon and nutrients before the wastewater is discharged to surface water or local wastewater treatment plants (EC, 2005). This has forced many industries to treat their wastewater to a level obtainable by implementing the best available technology for wastewater treatment.
Activated sludge (AS) processes have been applied worldwide in municipal and industrial wastewater treatment practice (Semblante et al., 2014; Devikarani et al., 2005) including applications on slaughterhouse wastewater (Chen and Lo, 2003). According to Semblante et al. (2014) the AS process involves the transformation of dissolved and suspended organic matters by a consortium of micro-organisms to gases and settlable biomass or sludge. While biological treatment offers high organic removal efficiency, it also entails significant sludge production, which contains active (live) and inactive (dead) micro-organisms and must be treated prior to disposal to prevent adverse impact on public health and the environment. Sludge production depends on different factors such as biodegradability of the organic pollutants, mass loading of the treatment plant, degradation rate of microbial cells by endogenous respiration or cellular lysis and existence of predator bacteria (Rocher et al., 1999).
Due to the nature of AS treatment and the slaughterhouses’ influent composition, a large amount of excess sludge is generated daily. Treatment and disposal of the excess sludge eats up 20-60% of the total operational cost of an...