FOG Bio-Augmentation Trial – Severn Trent

Executive summary

Fats, Oils and Greases (FOG) contribute to approximately 70% of the 40,000 blockages Severn Trent experiences each year. One of the reasons for FOG accumulation is due to Food Service Establishments (FSEs) disposing of fats and oils into the sewer. To avoid the build-up and hardening of FOG, Severn Trent invests in regular cleaning of its sewer network, which if not performed would cause blockages of the network and flooding. This work costs in the region of £10 million pounds per year. Severn Trent performed a 6-month trial at various locations testing NextFOGStop supplied by Next Filtration. NextFOGStop is a liquid solution that is composed of a protein-rich, cell-free supernatant obtained from a heat-shocked yeast mixed with a proprietary mixture of surface-active agents. The trial was conducted at various locations including sewers downstream of FSEs and pumping station wet wells and designed to evaluate the effect of NextFOGStop on FOG accumulation within the sewer. A range of criteria were used including CCTV assessment to categorise scores and condition grades, using the Water Research Centre (WRc) Sewer Condition Classification CCTV Coding; reduction in sewer blockages, sewer flooding events, proactive cleanses and an improvement in asset performance thus extending its life span.

The results for the trial were as follows:

• There was a delay in the buildup of FOG in the sewer downstream of FSEs compared to no dosing.
• Differences in the formation of FOG build up were observed with reduced hardening of FOG in sewage pumping station wet wells.
• The buildup of FOG was slower than when not dosed which provides opportunity for more efficient maintenance regimes.
• The effect of NextFOGStop in keeping FOG in a soft condition that is easily pumped away, led to lower energy consumption and much easier cleaning procedures for pumping stations.

The results have shown that the regular use of NextFOGStop could reduce the number of blockages and flooding events in the sewer network associated with FOG. During the period of the trial, while dosing NextFOGStop, the sites did not experience any blockages or flooding events.

Based on cost comparisons, the benefit of dosing NextFOGStop compared to business as usual, has the potential to reduce annual costs of cleaning a pumping station by 12.5%. Indications of financial benefits linked to risk avoidance for blockages and flooding events are contained within the full report.

The results from the 6 month trial are positive and the longer term benefits need to be monitored to fully understand the true reduction in maintenance cost and to understand whether the use of NextFOGStop establishes an equilibrium between the build-up of FOG and the degradation or removal of FOG by either the shear of the wastewater flow or the activity of the indigenous bacteria, that in itself avoids accumulation of FOG to cross sectional loss above 15-20% of the pipe.

We believe that the implementation of NextFOGStop in areas where source control is difficult will provide an alternative to frequent maintenance and will enable a more planned approach to be taken while maintaining or reducing costs.