Case Study 3 - A large garden centre complex

A garden centre has a wet well that collects all the water used on site but not rain water. The wet well then pumps via a 1,600 metre, 75 mm diameter pressure main to the gravity system in the nearest village. Several complaints had been received by the council relating to odour problems in the village and the sewers were clearly identified as the culprit.

The water services company installed hydrogen sulphide data loggers in the gravity system and identified that the most probable cause was the discharge of the pressure main from the garden centre. An Abatement Notice was duly served with significant financial penalties for non-compliance.

EnvEnt Ltd was called in to resolve the garden centre's problem. The daily waste water flows were calculated and the system was modelled; this confirmed that overnight when the garden centre was closed, the waste water could spend up to 20 hours in the pressure main.

Obviously the problem had been identified in the past as there was a small dosing system already installed that had originally used ferric sulphate. Unfortunately the dosing regime had been set to only dose when the sewage pumps ran. This was resulting in no dosing at all during the night when the centre was closed and inadequate dosing in the late afternoon when a maximum was required.

A new dosing pump has been installed and the system is now working correctly to maintain a suitable redox potential in the sewage to prevent hydrogen sulphide formation. More work will be required on this site in the summer as the waster water profile changes. EnvEnt Ltd has also kept the local authorities fully informed with progress and are now reassured that the problem is under control.