Performance of an expanded granular sludge bed (EGSB) reactor coupled with anoxic and aerobic bioreactors for treating poultry slaughterhouse wastewater

The generation of wastewater from slaughterhouses has become an environmental concern due to the growth of the poultry industry, as demand for poultry products has increased (Debik & Coskun 2009). Poultry slaughterhouses consume significant quantities of fresh water during slaughtering and cleaning of surfaces, resulting in the generation of a significant quantity of high strength wastewater (Debik & Coskun 2009), containing high organic matter, with high nitrogen and phosphorus constituents (Avula et al. 2009). The high phosphorous concentration is due to blood, cleaning and sanitizing agents, and the phosphorous is in the form of organic or inorganic phosphates (Arvanitoyannis & Ladas 2008; Avula et al. 2009). Furthermore, slaughterhouse wastewater contains a high quantity of biodegradable organic matter with a biological oxygen demand (BOD₅) range of 1.2 to 2.6 g/L, with the soluble fraction of the BOD, ranging between 40 to 60% (De Nardi et al. 2008, 2011). The primary pollutants contributing to the BOD in the poultry slaughterhouse wastewater are insoluble proteins from carcass debris, blood, fats, including non-biodegradable matter from feathers (Avula et al. 2009; Manjunath 2000; Yordanov 2010). The wastewater predominantly contains 35% more protein, resulting in a much higher BOD and COD, as compared to municipal sewerage (Zhang et al. 2008; Avula et al. 2009). Therefore, the poultry slaughterhouse wastewater should be treated efficiently prior to disposal to receiving fresh water sources to reduce environmental pollution (Debik & Coskun 2009).

Biological anaerobic treatment technology is one of the most recommended treatment methods worldwide in the treatment of wastewater from the food industry due to the technology's ability to treat high strength wastewater (Karnchanawong & Phajee 2009). Numerous research studies have focused on the application of a biological anaerobic digester for the treatment of poultry slaughterhouse wastewater (Avula et al. 2009). The feasibility of using a multi-stage process with an expanded granular sludge bed (EGSB) anaerobic digester coupled with anoxic and an aerobic reactor was examined in this study. Anaerobic digestion has been used in treating poultry slaughterhouse wastewater as it can efficiently handle variations in particulate matter and fats, oil and grease (FOG) loading rates. Anaerobic bioreactors such as an up-flow anaerobic sludge bed (UASB) reactor have been successfully used to treat poultry slaughterhouse wastewater. Del Nery et al. (2007) obtained treatment efficiency rates of 65% for total COD and 85% for soluble COD reduction at an average organic loading rate (OLR) of 1.64 kg COD/m³.day using a full scale UASB reactor. Similarly, (Debik & Coskun 2009) used a static granular bed reactor (SGBR) to treat the poultry slaughterhouse wastewater, achieving averaged COD removal rates of 95%. Similarly, the EGSB, which is well known to increase sludge expansion for improved efficiency due to its recirculation stream, was reported to achieve COD removal of 67% by Nunez & Martinez (2009), treating poultry slaughterhouse wastewater without a pretreatment process.

In this present study, a two-stage process containing an EGSB anaerobic digester coupled with anoxic and aerobic bioreactors was proposed to treat poultry slaughterhouse wastewater from a poultry product manufacturer in South Africa. From a South African perspective, the use of such a system has not been applied on an industrial scale, particularly for the treatment of poultry slaughterhouse wastewater. Therefore, this study was conducted to assess the performance of the bench-scale EGSB anaerobic bioreactor coupled with anoxic and aerobic bioreactors in treating the poultry slaughterhouse wastewater, to meet municipal discharge standard in South Africa.

The purpose of the EGSB reactor was to effectively reduce the organic load of the feed subsequent to the effluent being treated in the anoxic and aerobic bioreactors. The EGSB consisted of a cylindrical-shaped vessel with a total working volume of 1.2 L with a height and inner diameter of 0.22 m and 0.06 m, respectively. The reactor consisted of a gas–liquid separator at the top of the column for the separation of solids and biogas from the liquid phase. The biogas produced from the EGSB reactor was collected at the top of the reactor using Tedlar bags. The influent was pumped continuously and fed from the bottom of the reactor using a Gilson (Germany) peristaltic pump and the effluent was withdrawn from the top. The effluent liquid phase was split into two streams: (i) EGSB product and (ii) the recirculation stream. The recirculation stream from the top was mixed with the fresh effluent feed to the EGSB reactor. The liquid up-flow velocity was maintained at 1.1 m/hr. The reactor operated at a constant temperature of 37 °C, temperature regulated using a water jacket through which water from a thermostatic waterbath circulated. The reactor was also insulated to prevent heat losses to the environment.

The purpose of the anoxic tank and aerobic systems was for denitrification and nitrification processes, respectively. The anoxic and aerobic systems had a working volume of 0.825 L. The anoxic tank was placed on a magnetic stirring plate for continuous homogenization of the contents. The aerobic tank had two miniature air-diffusers with air supplied at a flow rate of 1.9 L/min.

The poultry wastewater was collected from an slaughterhouse located in the Western Cape Province, South Africa. The characteristics of the wastewater are summarized in Table 1, which lists averaged values of parameters quantified over a 3 week sampling period. All measurements were performed according to Standard Methods (APHA 2005).

The inoculum used for the EGSB was prepared as a mixture of 0.3 L granular sludge taken from a full-scale up-flow anaerobic-sludge bed (UASB) reactor treating brewery effluent (SABMiller, Newlands Brewery, South Africa) and 0.1 L of digested sewage sludge taken from a municipal wastewater treatment plant (City of Cape Town, South Africa). The anoxic and aerobic bioreactors were inoculated with 0.0825 L of digested sewage sludge and 0.7425 L of the poultry slaughterhouse wastewater.

During the start-up phase, the laboratory bench-scale EGSB was operated at an hydraulic retention times (HRT) of 7 days and an OLR of 0.5 g COD/L.day. Thereafter, the HRT was decreased each week to 4 days, and subsequently to 3 days, with OLR being increased to 0.7 g and 1.0 g COD/L.day, respectively.

Twenty five litres of fresh poultry slaughterhouse effluent was received every week and was analysed for pH, TDS, BOD, COD, NH₄⁺, TSS, TP, FOG and NO₃⁻. A sample was also sent to an independent accredited laboratory (Scientific services, City of Cape Town, SA) for full chemical analysis, for comparative analysis.

Analyses of the EGSB feed, EGSB product, anoxic and aerobic bioreactors product streams were monitored to assess the performance of the whole system, with values of parameters such as the chemical oxygen demand (COD), pH, being analyzed every 48hrs.

The system was started at an HRT of 7 days and OLR of 0.5 g COD/L.day to maintain start-up operating conditions, which provided the necessary acclimatization time for biomass and system stability. During the first week of operation, the EGSB experienced sludge washout due to high FOG and suspended solids loading in the feed. The sludge washout inhibited the EGSB performance. The following sub-sections describe and discuss the results obtained in the study.

The laboratory bench-scale EGSB anaerobic digester coupled with anoxic and aerobic activated sludge tank was used for this study to treat poultry slaughterhouse wastewater. Poultry slaughterhouse wastewater was treated at different HRT and OLR. The overall COD reduction for the system was 65%. The average EGSB reactor COD efficiency was 51%. The poor performance of the EGSB reactor was contributed to high FOG and suspended solids in the influent, which result in sludge washout and decrease in methanogenic activity. Sludge washout might have been the main contributor for poor biogas production in the EGSB reactor, as the methanogens floated out of the system. Due to reduce influent flowrate and sludge reduction during operation of the EGSB reactor, this study recommends that a pre-treatment system such as a Dissolve Air Flotation system be used to reduce the suspended solids and FOG in the feed. This will be beneficial to the performance of the EGSB reactor and thus improve its performance.

This study was supported by the National Research Foundation of South Africa (NRF), Thuthuka Funding.