Water Journal : Water Journal March 2011
sewer processes refereed paper technical features 84 MARCH 2011 water including the possible generation and release of ammonia gas. A dosing rig (Figure 2) was designed to control the addition of 50% caustic solution into the treated effluent flow, which was held at a minimum flow rate of 25ML/day for the duration of the trial. The Caustic Wash was performed during a weekend when the rehabilitation work was not being undertaken, so that the remainder of this effluent flow and the sewage flow could be diverted down the neighbouring sewer main. This increased the contact time while minimising the total amount of caustic required. Following the trial, the target pH and dose rate were optimised for maximum biofilm removal at minimum risk and cost. Results Trial Caustic Wash: pH profile Comprehensive pH monitoring was carried out at various locations along the LAP and receiving carriers, all the way to the receiving Malabar Wastewater Treatment Plant (WWTP). Many locations were monitored during this first trial to gain an understanding of how the pH is maintained through the system (Figure 3). The pH profile from this trial wash confirmed that the dosing regime was both operationally safe and practical. As seen in Figure 4, there was an instantaneous rise in pH as the caustic effluent reached scour valves 17 and 25, and the pH dropped rapidly once it had passed, indicating that the caustic effluent travelled mostly as a 'slug' while in the LAP pressure main, with little mixing or further dilution. It was noted that the pH only dropped slightly and the caustic was not significantly 'consumed' as it travelled through the LAP. Hence the only significant drop in pH was as the slug left the LAP and mixed with the other merging network flows. This was of crucial importance for the subsequent optimisation, as it placed restrictions on the maximum dose pH for future washes. Trial Caustic Wash: H2S profile Continuous monitoring of H2S gas at the outlet of the LAP enabled comparison of the H2S levels prior to and after the Caustic Wash. Even with a cautiously low pH (<11), there was a 30% average reduction in H2S levels, which lasted for a period of nine days before returning to original levels. The comparative chart presented in Figure 5 gives the average post-wash levels (blue), compared to an average of the pre-wash levels for similar days Figure 2: The dosing rig at Liverpool Wastewater Treatment Plant. Figure 3: Geographic overview of the LAP and the monitoring locations. Figure 4: pH in the network for the initial Caustic Wash.
Water Journal April 2011