Edgington, R.M., United Utilities Group PLC(free)
Abstract Davyhulme WwTW is United Utilities largest sewage and sludge treatment facility. The current sludge treatment facility is being expanded to include thermal hydrolysis and dewatering of the digested sludge with the production of ammonia rich liquors. For AMP5, the works has been given a tighter ammonia standard of 1mg/l (from the current 3mg/l) with predicted growth in population. Consequently, the treatment options for this centrate and along with the population growth (specifically the ammonia load) has been assessed. This has included both treatment within the sewage works including extensive trial work and a review of the available liquor treatment plant technologies.
Keywords: Centrate, liquor treatment, thermal hydrolysis, digestion.
Introduction to Davyhulme WwTW Davyhulme WwTW is United Utilities largest sewage and sludge treatment facility. The works serves the City of Manchester and parts of Greater Manchester, treating a population equivalent of about 1.2M. The works consists of two parallel streams based upon the activated sludge process which are both carbonaceous plants. The effluents from the two streams then combine to be treated on the BAFF (biological aerated flooded filter) for full nitrification. This is shown in Figure 1.
The current consent (on a 95%ile basis) is 30mg/l SS, 20mg/l BOD and 3mg/l ammonia and the final effluent is discharged to the Manchester Ship Canal.
The current sludge treatment facility at Davyhulme treats indigenous sludge and liquid sludge imports in conventional mesophilic anaerobic digesters (MADs) following gravity belt thickeners. The digested sludge is then pumped to the Shell Green incineration plant. During late 2012/early 2013, a thermal hydrolysis plant provided by CAMBI and associated ancillary equipment will be commissioned. Details of this new sludge treatment facility, SBAP (sludge balanced asset programme) have been covered elsewhere (Jolly, 2010). Following digestion, some of the sludge will be dewatered at Davyhulme for recycling to agriculture, while the remainder will be pumped to the Shell Green incinerator. Two dewatering centrifuges are being provided, each capable of producing 16000 tdspa of digested sludge cake. The current operational strategy is that either one or both of these centrifuges will operate depending upon the availability of the land bank for sludge recycling. The ammonia rich centrate from these centrifuges will have the excess solids removed by use of a DAF (dissolved air flotation) plant, followed by balancing. It will be returned to ASP1 stream for ammonia removal in the BAFF plant. Alkalinity dosing (sodium hydroxide) is being provided. An assessment by the SBAP project team showed that there is adequate capacity within the existing BAFF plant to treat this additional ammonia load and still meet the current consent of 3mg/l ammonia but not the future ammonia consent.