The Problem

This company already had an established effluent treatment plant to reduce COD and suspended solids prior to river discharge.

The system was based on a 2 stage treatment process combining settlement with anaerobic digestion.

COD reduction was poor resulting in high Mogden charges which typically were in the range of 60-65 p/m3.

The Solution

The first step was a detailed sampling programme by Vale Water personnel across the whole plant and to use the results to define the necessary changes to the system.

This analysis highlighted that the performance of the digester was the major contributor to the issues on site as follows:

•High level of suspended solids

•Absence of phosphate & low levels of nitrogen

•pH variable 5.5 – 6.2

•COD removal only in pre-acidification tank

Each aspect was then tackled individually with the overall objective in mind of improving the overall performance and to reduce Mogden costs.

The Result

On site working by Vale Water over a number of months resulted in Digester efficiency improving to such an extent that  gas production was at its highest level for over 2 years. This in turn translated into improved COD removal levels down from 2000 mg/l to 7/800 mg/l.

The presence of high sulphide odours around the plant had been a major issue for the site but with improved digester performance these have been eliminated completely.

The Future

The plant now achieves a sustainable performance of gas production including over the winter period and further work is on-going to introduce the “ValePac” series  of products to reduce further the COD levels.

The work undertaken by Vale Water has of course had a significant impact on the original Mogden charges which averaged 62 p/m3 equivalent to £545K pa with initial results achieving a reduction in Mogden charges to 51 p/m3 equivalent to an annual saving of over £100K pa.

Charges related to solids reduced to 48 p/m3 saving a further £30K pa Chemical costs averaged £35K pa so making the total saving of circa £95K pa.

The work continues on site with the objective of achieving potentially further improvements in COD removal and also reviewing and optimising the current processes in order to achieve further reductions in Mogden charges.