Advancements in poultry slaughterhouse wastewater treatment plant design

Abstract

The poultry industry is the largest agricultural sector in South Africa (SA), and it consumes large quantities of freshwater, which end up being discharged as high-strength poultry slaughterhouse wastewater (PSW). Generally, PSW is discharged into municipal wastewater drainage systems, which is detrimental for municipal treatment plants since a limited number of these wastewater treatment works (WWTWs) operate optimally. Furthermore, increasingly stringent standards for effluent discharge have placed an urgency on the development of advanced wastewater treatment technologies. Currently, extensive research has been done experimentally on lab-scale anaerobic and aerobic bioreactors; however, to upscale these reactors for industrial application, simulation and process modelling must be conducted to assess the feasibility of any proposed system. This study initially investigated the development of a model to simulate the performance of lab-scale anaerobic digesters treating PSW to assist small-scale poultry product producers. The single-stage, two-stage, and three-stage anaerobic digestion (AD) models were assessed with regards to predicting the removal of organic matter, total suspended solids (TSS) and volatile suspended solids (VSS) in PSW. However, from the model, there was a minuscule increase in nutrients, ammonia (NH3) and phosphate (PO43); thus, the model design required refinement. Thereafter, the performance of a lab-scale integrated multi-stage PSW treatment system consisting of an aerobic pre-treatment tank, an expanded granular sludge bed (EGSB) bioreactor coupled with submerged ultrafiltration (UF) membrane, with the objective being to assess the treatment efficiency of the individual treatment systems as well as that of the overall treatment system. The possibility of treating PSW to a water quality standard compliant with discharge by-laws or effluent discharge standards was investigated. The PSW used in this study was collected in 25L containers from a poultry slaughterhouse located in the Western Cape (WC) Province, SA and stored in a refrigerator at less than 4ºC until it was fed to the treatment plant. EcoflushTM, a hydrolysis agent, was added to the pre-treatment tank (25L) together with raw PSW, and the mixture was aerated for 24h using an adjustable air pump. The aerated mixture was then allowed to settle for a further 24h to reduce the mixture’s dissolved oxygen (DO) because the following treatment process was anaerobic. The pre-treatment process was batch operated at room temperature ranging between 20–23ºC, and on the third day, the resulting product was screened before being placed in a feeding tank (25L) that was continuously stirred using a magnetic stirrer. The EGSB (2L), containing glass marbles as the underdrain system, was inoculated with anaerobic granular sludge from a full-scale UASB reactor treating brewery wastewater, untreated PSW and a milk solution. The EGSB was operated continuously at mesophilic temperatures (33–40ºC) for 120 days. The membrane tank, which was also continually operated at ambient temperature (20–24ºC), was inoculated with raw PSW, tap water and EcoflushTM. In order to determine the reliability of the variations in the concentrations and removal efficiencies (REs) achieved, a visual outlier detection was implemented using boxplots. No outliers were detected for the membrane tank; therefore, data processing was only performed on the pre-treatment and EGSB processes. A correlation matrix using Heatmaps and density contours was applied to determine if there was a correlation between the REs investigated, and no correlations were observed. The pre-treatment process achieved REs of 44% chemical oxygen demand (COD), 66% fats, oil and grease (FOG) and 53% TSS, respectively, proving to be more effective at removing FOG due to the capability of the EcoflushTM hydrolysing the hydrocarbon chains in FOG. The organic loading rate (OLR) for the EGSB feed ranged from 200 to 700mgCOD/L.h. The EGSB successfully removed on average 56%, 63%, and 73% of the COD, FOG, and TSS present in the feed. The submerged membrane had the best RE performance obtaining on average 88% COD, 64% FOG and 90% TSS removal with an OLR that fluctuated between 50 and 450mgCOD/L.h. The integrated multi-stage treatment plant achieved overall average REs of 98% COD, 97% FOG, and 99% TSS reducing the content of the treated water to 101mg/L COD, 8mg/L FOG and 7mg/L TSS, i.e. qualities which are comparable to inland surface water, rendering it safe for discharge into the City of Cape Town (CCT) WWTWs.