Wessex finds Vitox an asset
The need to treat sludge press liquors on site and improve operating costs has led Wessex Water to adopt BOC Gases' Vitox technology at its Bridgwater and Wincanton STWs
Tankering relatively thin liquid off-site for final treatment is costly because the bulk of the load is water. Sludge transport costs can be reduced significantly by removing sufficient associated water at source. Sludge for pressing typically contains 4% dry solids and after pressing this can be increased to approximately 28-30%.
When Wessex Water decided to operate a sludge belt press on its Wincanton STW the company was faced with the problem of treating the resulting sludge press liquors. The existing works could not cope with the additional biological demand, and although the hydraulic input would have been negligible, the increased load on the existing biofilters would have been unacceptable. Sludge press liquors generally have higher BOD (biological oxygen demand) and ammonia levels than primary treated sewage. Wessex evaluated a number of process options before using an on-site system provided by BOC Gases.
Wessex chose an aeration system which required low capital and was straightforward to install. The Vitox option did not require any major civil work and brought a redundant capital asset back into use. A circular, steel tank, previously used for sludge storage, was converted to a stand-alone activated sludge plant with clarifier and sludge recycler. This made it possible to send the small volume of treated effluent to the existing works for final disposal.
The high anticipated rate of oxygen demand and the limited reactor volume available for oxygen mass transfer meant oxygen was perceived as the best available technology. Installation of a standard 2.4t/d oxygen Vitox drop-in unit was simple as the units are pre-assembled and self-contained. It just had to lowered in place and connected to utility supplies. The need for the plant to nitrify meant dissolved oxygen, greater than 1.0mg/l, had to be available from the outset. The single portable drop-in unit also provided, in conjunction with a small submersible mixer, sufficient momentum to mix the reactor contents and ensure homogeneity.
The recent conversion of a Unox high-rate pure oxygen, activated sludge plant at Wessex Water's Bridgwater STW to Vitox has resulted in 25% savings in oxygen consumption and improved process mixing. The Unox process was developed by Union Carbide and consists of a covered tank divided into a series of pockets, normally three. A pure oxygen atmosphere is created in the head-space by the addition of high-purity oxygen under slight positive pressure. Oxygen mass transfer is achieved by mechanical agitation in each pocket.
The oxygen and sewage run concurrently through the reactor. Carbon dioxide levels increase throughout each successive section in the reactor, which causes the efficiency of oxygen transfer to fall gradually. The exhaust gas from the system contains a mixture of nitrogen, carbon dioxide and oxygen. Oxygen concentration levels were typically in the order of 30-40% before the conversion to Vitox. BOC Gases' Vitox process is a high-rate pure oxygen jet dissolver capable of high oxygen transfer efficiencies and intimate process mixing. Higher dissolution efficiencies are due to the smaller gas bubble size created by the highly turbulent Vitox jets. Improving the oxygen transfer efficiency by retrofitting Vitox as the principal mass transfer device in the front section is projected to show a net financial benefit.
Cut and mix
Improved operating cost was one of the main reasons for adopting Vitox, but process benefits have also resulted. By switching to a sub-surface jet dissolver, better mixing in the front pocket was observed on start-up. The Unox surface aerators had limited mixing capabilities resulting in the deposition of some biosolids in the front pocket. The gradual reduction in reactor volume would have lowered the hydraulic retention time available for biotreatment and may have eventually impaired pollution removal efficiency.
Since the Vitox equipment was installed in both Unox reactors, the plant has performed well, achieving all its final effluent quality targets. An increase in oxygen dissolution efficiency will enable the operators to increase the capacity of the plant or operate under the same loading regime for lower overall operating costs. Nigel Coram, leading operator at the site, noted that: "Tanker movements have been noticeably reduced since the Vitox equipment was installed".