Evaluación de riesgos

Few wastewater treatment plants (WWTP) can treat PSW completely in a single stage. Often, when there are challenging wastewater treatment (WWT) targets, two or more sequential treatment stages are required as discussed in 2.3. PSW has been treated using physical, chemical and biological processes prior to discharge into receiving surface water sources. This is due to its relatively biodegradable characteristics (BOD/COD > 0.5). It has been suggested that an anaerobic biological treatment process is one of the most suitable and effective treatment processes available (Cao and Mehrav, 2011). However, prior to using this type of biological treatment process, an efficient pre-treatment system such as a suspended solids separator is needed, as the wastewater contains a high concentration of TSS and FOG which can lead to the failure or instability of the biological treatment process (Manjunath et al., 2000).

Other pre-treatment systems are used to remove TSS and FOG from the influent prior to the downstream treatment of the PSW, such as grease-traps, tilted plate separators, or a DAF system supported by chemical and/or biological agents. However, this increases operational costs due to the reagents and personnel needed to operate numerous process units (Del Nery, 2007). It can be advantageous to use pre-treatment methods to minimise sludge flotation and the clogging of PSW treatment units which is caused by the FOG, feathers and blood (Del Nery, 2007). Prior to PSW discharge from the WWTP, poultry product processors are required to remove a majority of the soluble and particulate organic matter present in the wastewater in order to achieve compliance with environmental discharge regulations. The majority of poultry processors use some form of screening application to reduce suspended particulates, including internally and externally fed rotary screens, shakers and bar type screens (Avula et al., 2009).

Most PSW treatment systems utilize activated sludge in anaerobic reactors as their primary biological treatment stage. The high energy demand requirements for aeration of aerobic reactors, considering the large quantity of sludge generated, increases overall disposal costs of the excess sludge for WWTPs. This therefore limits the potential of aerobic technology as the primary biological treatment stage of high strength industrial wastewater such as PSW. The energy savings and mitigation of unnecessary activities associated with anaerobic digestion processes can culminate in minute excess sludge production, thus reducing disposal costs. Del Nery et al., (2007) showed that utilizing a combination of reactors, i.e. an up-flow anaerobic sludge blanket reactor (UASB) and a stirred tank reactor coupled with a membrane filtration unit, achieved >90% organic matter removal. This treatment strategy can be adopted with minimal modification to treat PSW collected for this study. Possibly to a similar degree of effectiveness. According to Avula et al. (2009), some physical methods

have also been reported in the reviewed literature as effective, with 3 major categories being identified, namely: (a) the destruction of pollutants by an electrical change or UV radiation, (b) the combination of biochemical and chemical destruction using oxidants such as ozone, chemical separation or biochemical degradation systems, and (c) physical separation processes using technology such as DAF and membrane filtration systems. However, all these processes produce recalcitrant by-products which can further contaminate available natural water sources. According to Avula et al. (2009), the advantage of existing physical PSW treatment processes are: (1) only minimal efforts can be made to reclaim nutrients, and (2) other valuable constituents in PSW can be degraded during the biological treatment process to produce biogas. Previous studies of PSW treatment included the utilisation of a DAF and a UASB. According to Del Nery et al. (2008), treatment comprising a DAF system and two UASB reactors in series can achieve complete organic matter degradation rates. A full-scale DAF system was determined to accomplish unsatisfactory removal efficiencies of 15% for SS and only 8% for FOG, suggesting operational inadequacies (Del Nery et al., 2008). However, a lab scale DAF system showed that flocculation agent addition and the implementation of air pressurization, including a 40% recycled effluent can increase TSS and FOG removal by up to 74% and 99% respectively (Del Nery et al., 2008). Furthermore, a study by Del Nery et al. (2007) in which long-term operation performance was monitored over 4 years using rotary and static screens, an equalization tank, a DAF system and two UASB reactors showed an average of 51% FOG removal and 37% TSS removal. The operational parameters of this system included an OLR, applied to the UASB that ranged between 0.9 to 2.7 kg tCOD/m3.day with up-flow velocities varying from 0.2 to 0.5 m/h. The system showed the satisfactory performance of UASB reactors, with a tCOD removal efficiency of 85% (Del Nery et al., 2007). For the recovery of essential compounds, recovery processes can be implemented; for example, Lo et al. (2005) recovered protein from PSW using membrane ultrafiltration after the primary treatment stage, which included two DAF systems in parallel for the removal of 90% of FOG in the PSW. This resulted in retainment of the crude proteins, which subsequently reduced the tCOD to less than 200 mg/L. However, the membranes were fouled severely, resulting in the implementation of cleaning-in-place processes to restore performance (Lo et al., 2005).

For this current study, a lab-scale expanded granular sludge bed reactor (EGSB) was used as the primary anaerobic digester, with filtration implemented as a pre-treatment stage, followed by a post EGSB treatment using single-stage nitrification and aerobic denitrification process attached to a hybrid side-stream UF membrane bioreactor (MBR).