Multiple use of iron in an urban water system

Abstract

Water and wastewater treatment chemicals specifically coagulants such as iron salts are used in many different process units of the urban water system, including water treatment plants, sewer networks, wastewater treatment plants, and anaerobic digesters. The use of iron salts is mainly driven by each given process unit without the consideration of the flow-on effects to the downstream systems. This work aims to demonstrate the benefits of the multiple re-uses of iron in the urban water system, through the development of a laboratory scale reactor system simulating the integrated urban wastewater system. This integrated and innovative strategy will set an example of how to achieve major environmental and economic benefits through the holistic management of urban water and wastewater operations. Specifically, this thesis investigated the (1) effects of sewer-dosed iron salts on the wastewater treatment process through a comprehensive laboratory assessment using the reactor system, (2) the effects of in-sewer iron rich drinking water sludge on wastewater collection and treatment systems, and (3) the impact of primary sedimentation on the multiple use of iron in the urban wastewater system.A comprehensive laboratory assessment of the effects of iron-dosing in the sewer system on the wastewater treatment process was investigated in Chapter 5. A developed laboratory scale system comprising sewer reactors, wastewater treatment reactors. Sludge thickeners and anaerobic digesters were used. Two systems, fed with real domestic wastewater, were operated for over a year. The experimental system received ferric chloride (FeCl3) dosing at 10 mgFe L-1 in the sewer reactor whereas the control system received none. Wastewater, sludge, and biogas were extensively sampled and analysed for relevant parameters. The FeCl3-dosed experimental system displayed a decreased sulfide concentration (by 4.3 ± 0.5 mgS L-1) in sewer effluent, decreased phosphate concentration (by 4.7 ± 0.5 mgP L-1) in biological treatment reactor effluent, and decreased hydrogen sulfide concentration in biogas (911.5 ± 189.9 ppm to 130.0 ± 5.9 ppm), as compared with the control system. The biological nitrogen removal performance of the treatment reactor and biogas production in the anaerobic digester was not affected by FeCl3-dosing. Furthermore, the dewaterability of the anaerobically digested sludge was enhanced by 17.7 ± 1.0 %. These findings demonstrate that iron-dosing to sewers can achieve multiple benefits including sulfide removal in sewers, phosphorus removal during wastewater treatment, and hydrogen sulfide (H2S) removal during biogas generation.Drinking water production that uses FeCl3 as a coagulant produces waste sludge rich in iron. We hypothesised that the iron-rich drinking water sludge (DWS) can potentially be used in the urban wastewater system to reduce dissolved sulfide in sewer systems, aid phosphate removal in wastewater treatment and reduce hydrogen sulfide in the anaerobic digester biogas as discussed in Chapter 6. This hypothesis was investigated using two laboratory-scale urban wastewater systems, one as an experimental system and the other as a control, each comprising sewer reactors, a sequencing batch reactor (SBR) for wastewater treatment, sludge thickeners and anaerobic digestion (AD) reactors. Both were fed with domestic wastewater. The experimental system received in-sewer DWS-dosing at 10 mgFe L-1 while the control had none. The sulfide concentration in the experimental sewer effluent decreased by 3.5 ± 0.2 mgS L-1 as compared with the control, while the phosphate concentration decreased by 3.6 ± 0.3 mgP L-1 after biological wastewater treatment in the experimental SBR. The dissolved sulfide concentration in the experimental anaerobic digester also decreased by 15.9 ± 0.9 mgS L-1 following the DWS-dosing to the sewer reactors. The DWS-doing also enhanced the settleability of the mixed liquor suspended sludge (MLSS) (SVI decreased from 193.2 ± 22.2 to 108.0 ± 7.7 ml g-1), and the dewaterability of the anaerobically digested sludge (the cake solids concentration increased from 15.7 ± 0.3 % to 19.1 ± 1.8 %). The introduction of DWS into the experimental system significantly increased the COD and TSS concentrations in the wastewater, and consequently the MLSS concentration in the SBR, however, this did not affect normal operation. The results demonstrated that iron-rich waste sludge from drinking water production can be used in the urban wastewater system achieving multiple benefits.The in-sewer dosing of iron rich DWS strategy was further investigated using a different WWTP configuration. In Chapter 7, the impact of primary sedimentation on the multiple uses of iron in the urban wastewater system was investigated. Following the operational procedure for DWS-dosing but with primary sedimentation operation for both control and experimental systems. It was hypothesised that primary sedimentation could significantly change the characteristics of the wastewater flowing into the wastewater treatment reactor, particularly the solids content. This could affect the availability of iron for further beneficial effects in wastewater treatment. Around 85% of the Fe in the sewer effluent was found in the primary sludge and the remaining the PS tank effluent. The flow-on effect of Fe was significantly reduced in wastewater treatment removing only 1.2 ± 0.1 mgP L-1, as compared to 3.5 ± 0.1 mgP L-1 achieved in the absence of a primary settler. The P to Fe removal ratio in this study was 0.32 mgP/mgFe, similar to the ratio observed without primary sedimentation (0.36 mgP/mgFe). The dissolved sulfide removal in the AD decrease removing only 2.7 ± 0.5 mgS L-1 in comparison to 7.2 ± 0.3 mgS L-1 removed without primary sedimentation. suggesting that Fe was only partially available for dissolved sulfide removal. However, the dewaterability of the anaerobically digested sludge relatively improved by 24.8 ± 2.2%. The in-sewer DWS dosing strategy still has its benefits even with primary sedimentation in operation, controlling sulfide in sewers, and improving digested sludge dewaterability.In summary, the results of this PhD thesis provided deeper understanding and information necessary to demonstrate the multiple re-use of iron in the urban water system.  The use of iron salts demonstrated the multiple re-use of iron in the urban wastewater system. The use of iron rich DWS is as effective as the use of iron salts with minimal effects on normal operation. Primary sedimentation significantly affects the delivery of multiple benefits of iron in the urban wastewater system. This concept provides more efficient use of iron salts and promotes integrated management of the urban water system.