Solids-Liquid-Gas (SLG) separation technology for sludge treatment: Results from a pilot scale trial by Anglian Water

Inman, D.1, Capeau, P.2, 1Anglian Water Services (UK), 2Orege, France



Anglian Water has a large number of small rural water recycling centres spread over a large area, each of which produces sludge that needs to be transported to centralised sludge treatment centres. Removing water from that sludge prior to transportation reduces costs and CO2 emissions contributing to sustainability and efficiency goals. Existing technology used for thickening (gravity belt thickeners) and dewatering (centrifuges) achieve liquid-solids separation using filtration or centrifugation, and are limited by the characteristics of the sludge. In the pilot trial reported below, changing the characteristics of sludge by preconditioning is considered as a way to improve the performance of these existing assets.

The SLG process is a novel approach that improves the dewaterability of sludges by removing bound water through the injection of pressurized air. Air is diffused into the sludge at the same time to aid flocculation, leading to a denser floc and easier to dewater sludge.

The process was tested under controlled conditions at Flag Fen Water Recycling Centre in Peterborough, as a pre-conditioning step prior to thickeners and centrifuges. The trial also tested the use of the SLG for solids flotation as an alternative to membrane separation. The results demonstrate that the SLG process can improve dewaterability of wastewater sludge, with potential to significantly improvement the treatment of surplus activated sludge (SAS).

Keywords: Orege SLG, sludge, thickening, dewatering, municipal sludge, biosolids, innovation, surplus activated sludge


Efficient thickening and dewatering of wastewater sludges remains one of the key challenges for UK water companies. Biological sludge from water recycling is known to have properties that make it difficult to dewater such as high bound water content and compressibility (Sorensen and Hansen, 1993; Katsiris and Katsiri, 1987). In Anglian Water, the preferred approach is either to thicken to a 3 to 6% sludge using a drum or belt thickener, or to produce a 20 to 25% cake using a centrifuge.

In terms of the overall costs associated with operating a Water Recycling Centre (WRC), the cost of sludge treatment and handling is second only to the electricity used for secondary aeration. The majority of this expense is the movement of the treated and dewatered sludge. For this reason, WRC managers are constantly seeking to reduce solids handling costs, but it is a challenge to find the optimal balance between the amount of polymer used and the cake dryness. Many plant operators believe this can only be achieved by switching to alternative dewatering equipment. Such a significant change presents additional challenges as a plant operations team is forced to learn an entirely new technology and the facility may incur new operating costs in certain areas (e.g. staffing, energy, maintenance & repair) as they endeavour to reduce polymer usage and increase cake dryness. Treatment plant operators would ideally install new equipment that uses less polymer usage and produces a cake with higher dry solids content; however the high costs of replacing less efficient equipment usually makes complete replacement economically unfeasible. An alternative approach, presented in this paper, is to utilize existing dewatering equipment, but make it operate more efficiently by adding a pre-treatment step to improve the dewaterability of the sludge.

A new technology, developed by Orege in France, called the SLG™ (Solid-Liquid-Gas) process claims to be able to improve the operational efficiency of existing thickening and dewatering equipment. The process acts as a ‘sludge conditioning’ step, upstream of existing equipment, and by changing the characteristics of the solids, improves its dewaterability.

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