Post Project Appraisal of Liming of Raw Sludges

Taylor, C. and Edgington, R.M., United Utilities Group PLC



Three sludge treatment centres in United Utilities have had lime dosing systems installed as part of a capital programme in AMP4. The processes are now configured so that hydrated lime is added to raw liquid sludge prior to being dewatered by centrifuges. This has enabled each treatment centre to achieve treated sludge through a 2 log kill of Escherichia Coli allowing the sludge to be recycled to land. Each site has been revisited and an appraisal carried out to review the performance post commissioning. The benefits and shortfalls since the three projects have been completed are identified with the key learning points being highlighted.

Keywords: E.Coli, Liming, Liquid, Log, Kill, pH, Sludge


In AMP4 a number of Sludge Treatment Centres (STCs) required upgrading to ensure that sludge achieved a treated standard to ensure that bio-solids disposal to agricultural land was viable. Three of the sites identified in this driver to achieve a treated sludge standard were: Fazakerley, Carlisle and Workington STCs. The preferred solution at each site was to install a liquid liming process to dose hydrated lime directly into raw liquid sludge. The limed raw sludge is allowed to mature for a predetermined interval at a fixed pH to achieve the 2 log kill of Escherichia Coli (E.Coli) required for a treated standard as per the Safe Sludge Matrix (Water UK, 2010). Following the maturation period, the sludge is dewatered by centrifuges producing a cake for agricultural land and the centrate liquors are returned to the Wastewater Treatment Works (WwTW) for treatment. A generic Process Flow Diagram (PFD) is provided in Figure 1. Site specific processes are described later in the paper.

Treatment Works Description Fazakerley WwTW is located North West of Liverpool adjacent to the River Alt in the United Utilities (UU) Merseyside Region treating crude sewage from a Population Equivalent (PE) of 164,254. The works has a low level inlet which is primarily trade load and a high level inlet from domestic residents. The low level works consists of Primary Settlement Tanks (PSTs), high rate filters and intermediate settlement. The sludge from the primary and intermediate settlement tanks goes directly to the STC. The high level works has PSTs only before it combines with the low level works. Following amalgamation of the two works there are trickling filters, humus settlement tanks and biological flooded filters. Humus sludge is returned to the PSTs for cosettlement of solids. All sludge processed at Fazakerley STC is indigenous sludge.

Carlisle WwTW is located in the centre of Carlisle on the banks of the River Eden in the UU North Cumbria Region treating crude sewage from a PE of 110,387. The works has trickling filters employing co-settlement of solids in the PSTs. Carlisle STC also receives imports from a number of smaller works in the surrounding area, it is estimated that the indigenous sludge to imported ratio is 60:40.

Workington WwTW is also located in the UU North Cumbria Region and is North of Workington between Northside and Siddick treating crude sewage from a PE of 54,332. The works has an activated sludge plant with co-settlement of solids in PSTs. Workington STC, like Carlisle, also receives imports from a number of smaller works in the nearby area, it is estimated that the indigenous sludge to imported ratio is 50:50.

As part of the post project appraisal process operational data was examined and the performance of each of the STCs, in terms of E.Coli log kill and dewatering, was calculated; this is discussed in turn for each STC. Another important part of the appraisal was to visit each STC and discuss, with the operational staff, the operability of the new process to highlight any key lessons learnt.

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