Identifying the Source, Cause, and Solution of Biosolids Odor at a Maryland Water Resource Recovery Facility

INTRODUCTION Recently, the Western Branch (WB) Water Resource Recovery Facility (WRRF), one of the five major WRRFs of Washington Suburban Sanitary Commission (WSSC), has the immediate challenge with the biosolids odor produced during the processes of storage, dewatering, and transport of biosolids. The dewatered biosolid cake in WB-WRRF is disposed in landfills now, but the landfills have expressed concerns over excessive odors and in some instances stopped accepting the dewatered biosolid cake. This problem will be continued for the next 3 years. WSSC is looking for some solutions to reduce the biosolids odor issue. According to the processes operated in WB-WRRF, there are several possible reasons causing the odor emission issue. First of all, the blending of sludges with different characteristics may contribute to biosolids odor development. At WB-WRRF, three separate streams of waste sludge, namely high-rate activated sludge (HRAS), nitrification activated sludge (NAS), and denitrification activated sludge (DNAS) are combined in WAS Wet-Well and thickened using a dissolved air flotation (DAF) system. It is likely that the combination of these sludges has augmented the biosolids odor production. Second, the anaerobic condition created during the sludge holding time in the sludge holding tank may be another possible factor contributing to the biosolids odor emission issue. The dewatering centrifuges were shut down during the weekend, leading to 15 ft deep blended sludge accumulation in the two holding tanks for around 48 hours. The bottom of the holding tank may be considered anaerobic, and many offensive odor compounds could be produced during the holding time. It was reported that the major odor-causing compounds emitting from anaerobically digested sludge are mainly H2S, and volatile organic sulfur compounds (VOSCs) including methanethiol (MT) and dimethyl sulfide (DMS) (Novak et al., 2006). Therefore, it is our hypothesis that process such as aeration may maintain a relatively high oxidization reduction potential (ORP) in the sludge holding tanks and mitigate the odor generation. The objectives of this research include: 1. Measure ORP profiles throughout all treatment trains in WB-WRRF to infer the suspicious spots that have high odor generation potential. 2. Test various combinations of the blending of HRAS, NAS, and DNAS to study their effects on the odor emission following sludge dewatering. 3. Evaluate the effect of aeration in the sludge holding tank on odor emission from dewatered cake. METHODS Fresh HRAS, NAS, DNAS and blended sludge were collected from WB-WRRF and blended based on the actual blending ratios used in the WB-WRRF. A laboratory dewatering protocol was established to mimic the full-scale centrifuge dewatering processes following three key steps: (1) polymer conditioning under controlled mechanical shearing at G t value of 105, where G is the mean velocity gradient and t is the retention time taken for sludge to be exposed to shear force; (2) centrifugal sedimentation using a lab centrifuge; (3) cake compressing using a piston under controlled pressure to obtain a cake around 20% total solids which is similar to that in the full scale. The dewatered cake was stored in a glass jar with a septum stopper. The contents of odor compounds such as H2S, MT, DMS in the headspace of the jar were measured using a gas chromatograph (GC). Headspace gas samples were collected on a timely basis and manually injected into the GC. Meanwhile, the ORP profiles across the entire WB-WRRF were measured using two ORP probes. RESULTS ORP profiles along with the treatment trains of WB-WRRF. The three types of sludge in WB-WRRF, namely HRAS, NAS, and DNAS, were firstly combined in a WAS wet-well and then thickened in DAF prior to being stored in holding tanks in preparation for dewatering. As shown in Figure 1, we measured the ORP profiles along with these treatment tanks and found that the ORP quickly dropped from 113 mV in HRAS reaction tank to -81.55 mV in WAS wet-well. Thereafter, the ORP further dropped to -218.95 mV in the sludge holding tank, creating ideal conditions needed for formation of odorous compounds such as H2S, MT and DMS. High-rate activated sludge is the major source of odor generation The headspace peak concentrations of H2S, MT, and DMS measured during the storage time for seven combinations of the three types of sludge are shown in Figure 2. As can be seen, whenever HRAS was blended in, the peak concentrations of the sulfur-containing odorous gas were always prominently high. In contrast, only negligible peak concentrations of H2S, MT, and DMS were observed in NAS, DNAS, and NAS + DNAS. Since the blending ratios used in this study were based on the actual blending ratio in the WB-WRRF, it can be concluded that HRAS accounts for the odor generation during the dewatered cake storage, and NAS and DNAS appear to have little to do with odor generation. Aeration effect on the odor emission from dewatered cake (bench-scale). Aeration of thickened solids in the sludge holding tank was tested at bench-scale as an odor mitigation strategy. As can be seen in Figure 3, all control groups and the mechanical mixing groups demonstrated the higher peak concentrations of odorous compounds than corresponding aeration groups during the storage time (Figure 3). Particularly, the cake from the control group had the highest MT peak concentration, while the aeration group had the lowest one (Figure 3b). Likewise, the aerated sample gave the lowest DMS level (13.86 mg m-3 g-1) which is approximately 30% lower than those in the other two conditions (Figure 3c). Undoubtedly, the aeration in the simulated holding tanks substantially attenuated the extent of H2S, MT, and DMS emission from dewatered cake. Aeration effect on the odor emission from dewatered cake (full-scale). After obtaining the favorable result in the bench-scale SHTs aeration test, full-scale testing of aeration in SHTs as an odor mitigation strategy was conducted, as well. As shown in figure 4, the dewatered sludge with aeration pretreatment during the sludge holding time in the SHTs generated less H2S than the group without aeration pretreatment during the storage time (Figure 4a). There was not much difference between these two groups in terms of MT concentration (Figure 4b). Moreover, only minor levels of DMS were produced (Figure 4c), indicating that the major odorous compounds emitted from the full-scale dewatered cake were H2S and MT. We can conclude that aeration during the sludge holding time can reduce odorous compounds generated from the dewatered cake.