Water Journal : Water Journal November 2012-1
refereed paper odour management water NOVEMBER 2012 67 Scrubbing and Activated Carbon Filtration). The selected configuration in chemical scrubbing (one stage versus two stages) significantly influences the investment costs, with two-stage chemical scrubbers presenting higher costs than their one-stage counterparts. Nowadays, odour removal at typical design efficiencies of 90–95% demands at least two stages for higher odour loads, whereas one stage is enough when only H2S abatement (95–99%) is required. The investment costs in Activated Sludge Diffusion are not shown, as these would be minimal because all equipment required is already present in the wastewater treatment line. Additional investments for Activated Sludge Diffusion would derive from the installation of moisture traps and dust and grease aerosol filters, and from the use of corrosion-resistant materials in blowers and air piping. In this context, a survey from 30 WWTPs in the US showed that corrosion concerns were not well founded; but in most cases, corrosion- resistant materials must be installed in filters, moisture traps and blowers (Bowker and Burgess, 2001). Despite its limitation in air volume and foul air loading, Activated Sludge Diffusion is the cheapest of all the technologies selected. During evaluation and selection of odour abatement systems, the Net Present Value (NPV) rather than the initial investment cost must be used as economic selection criterion. For a relatively low (7,500m3 h-1) odorous emission containing less than 7.5 ppmv of H2S, Activated Carbon Filtration is the cheapest technology, but the dearest when the H2S exceeds approximately 20 ppmv . (Figure 2a). For higher airflows (75,000m3 h-1) it reveals that the biotechniques (Biofiltration, Biotrickling Filtration as well as the Hybrid Technology) become increasingly less expensive (NPV20) with increased air flows (Figure 2b). The cost of secondary effluent water is assumed to be less than potable water (as secondary effluent is often used as plant service water), which favours the operational costs and the overall costs (NPV20) of the biotechniques. Nearly complete (>99%) H2S degradation in the BTF stage was assumed for the Hybrid Technology. The effect of higher H2S concentrations on Biofiltration is typically a reduction of biofilter media lifespan due to an increased acidification of the BF media, which reduces its abatement performance. Therefore, a biofilter media lifespan of five years at an average H2S concentration of 5 ppmv , and a media lifespan of two years at 40 ppmv was used in this evaluation. A single-stage chemical scrubber offers for relatively low concentrations an opportunity to reduce the capital costs of Chemical Scrubbing. A single- stage chemical scrubber could reduce the investment cost by about 40% (data not shown). This cost saving on investment will result in an approximately 10% saving from the total costs (NPV20) at an airflow of 7,500m3 h-1 and an inlet concentration of 20 ppmv H2S (app. 5% at 75,000 m3 h-1 ), and an approximately 15% saving from the total costs (NPV20) at an inlet concentration of 5 ppm (app. 10% at 75,000 m3 h-1 ). To obtain the assumed required reduction of 95% in odour concentration, both caustic and hypochlorite dosing are still needed, which typically results in slightly higher chemicals consumption in the single- stage scrubber compared to a two-stage scrubber. In summary, although capital cost savings for a single-stage scrubber is significant, the overall cost savings (NPV20) are not as large compared to a two-stage scrubber to obtain 95% odour removal, because the total costs of Chemical Scrubbing are mainly determined by the operating costs. The influence of the utility prices (energy and labour) and design parameters (media life and reactor size) on the NPV20 of the five odour abatement technologies are evaluated. The biological technologies (Biofiltration and Biotrickling Filtration) shows the lowest operating costs as illustrated by Estrade et al. (2012). Figure 3a shows that at 50% higher energy prices compared to the currently used $0.15 per kW, Activated Carbon is the least affected, as energy is only a very small part of all operating costs, while Biofiltration is most affected followed by Chemical Scrubbing and the Hybrid Technology. Figure 3b illustrates that when the labour cost is increased by 25%, Activated Carbon and Biofiltration are most affected due to their relatively low media lifespan and large volumes of Figure 1. The investment costs for Activated Carbon Filtration, Chemical Scrubbing, Biofiltration and Biotrickling Filtration (adapted from Estrada et al., 2011). Figures 2a and 2b. The Net Present Value (NPV20) evaluated for a 7,500m3 h-1 odorous emission (Figure 2a) and evaluated for a 75,000m3 h-1 odorous emission (Figure 2b) at different H2S-concentrations. CS is Chemical Scrubber, AC is Activated Carbon, BF is Biofiltration, BTF is Biotrickling Filtration, BTF+AC is Biotrickling Filtration with Activated Carbon polishing.
Water Journal December 2012
Water Journal September 2012-1