Water Journal : Water Journal March 2011
refereed paper sewer processes water MARCH 2011 83 Abstract The Liverpool-Ashfield Pipeline (LAP) is a 24km pressure main of 1m diameter, and was being used temporarily to divert raw sewage to allow for intensive rehabilitation works on a nearby major sewer main. 'Caustic Washes' were performed using a 'shock-dose' of sodium hydroxide to remove the biofilm from the pipe walls. A reduction in hydrogen sulphide gas (H2S) was achieved, with levels decreased by 70% and maintainable at this level for a period well in excess of 20 days. It was found that caustic soda washing is viable as an additional treatment process, offering improved control over the elevated H2S levels and, most importantly, offering the additional benefit of control over methane generation. For an optimised and compounded set of washes, the results were found to last for up to several months. This alternative method of odour mitigation is portable to other similarly structured networks. Introduction The Liverpool-Ashfield Pipeline (LAP) is a 24km pressure main of 1m diameter, now conveying treated effluent for industrial re-use. However, for two years until 2010 it was pressed into service as a diversion for raw sewage during rehabilitation of a major sewage main, the Northern Georges River Submain. The main challenges stemmed from the fact that it is a very long, large-diameter pressure main, and due to the presence of a well-developed biofilm, the LAP acted as a large biological reactor. During warmer summer months, levels of methane (CH4) and hydrogen sulfide (H2S) gases at the discharge point of the LAP were particularly elevated, especially after the LAP had been shut down for a period of time. It has a turbulent vortex chamber at the drop shaft where the LAP discharges into a gravity sewer main approximately 20 metres below. At this point, a bio-trickling filter and activated carbon unit were in place to treat the odorous air, due to the close proximity to sensitive residential housing (Figure 1). A temporary solution to manage the high H2S was to install an additional activated carbon unit to help reduce the sulfide peaks, but this offered no mitigation over the methane generated in the network. Chemical dosing of ferrous chloride was used (due to the long detention time in the LAP) to condition the sewage, somewhat reducing the amount of H2S gas released. This provided limited control of gaseous sulfide generation, however, it did not decrease methane generation. Due to the biofilm-enhanced H2S and CH4 generation in this system, it was proposed that even maximum ferrous chloride dosing and optimised pumping operations are not adequate to efficiently deal with this problem during the warmer summer months. This meant that an alternative and additional means of odour control was required. Aim of the Project The aim of the project was to reduce the H2S and CH4 levels at the LAP outlet. This was to be achieved by removal of the biofilm present in the LAP, which was the principal cause of the elevated gas levels. Development of a safe and repeatable method of treatment was necessary, involving laboratory and field trials with in-depth monitoring and analysis. Operational, occupational health & safety, environmental and stakeholder concerns were to be considered. A secondary aim was to consider the validity of applying this washing methodology as part of the cleaning process when converting the LAP back to its intended use as a recycled water main. Method Used Elevated pH using sodium hydroxide (caustic soda) was identified, through literature review, as the most suitable means of biofilm removal. A shock- dose was employed, utilising a higher pH for a short period of time in order to attack and remove the biofilm. This is quite different to the common method of employing a sustained, slightly elevated pH (such as with magnesium hydroxide), which simply serves to keep sulfides in the aqueous phase due to the pH equilibrium. From a combination of laboratory trials, stakeholder consultation and a comprehensive risk assessment, the initial trial conditions were determined, using a lower pH (<11) with a contact time of six hours. This was done to minimise the risks to the downstream network, J O'Gorman, I Purssell, G Iori Cost-effective removal of biofilm that reduced both odour and methane. CAUSTIC SODA WASHING OF A SEWER PRESSURE MAIN Figure 1: Proximity of the air treatment facility to residential housing.
Water Journal April 2011