Case Study 2 - A small package plant treating a village and a leisure complex.

EnvEnt Ltd was called in by a major water service company to diagnose and solve an odour problem related to a new package plant that had been recently commissioned. The primary task was to eliminate foul odours from the works which were the cause of complaints in the village.

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The new plant was composed of two identical package units comprising a primary sedimentation zone, rotating bio-contactors (RBCs) and a final humus tank. The units were enclosed in GRP housings. Design capacity was about 2,000 Person Equivalents.

Some initial measurement and inspection showed low redox potentials in the primary tanks, up to 35 ppm of hydrogen sulphide in air inside the GRP housings, and obvious signs of sulphur cycle bacteria on the first three discs of the bio-contactors. There was already significant corrosion of all exposed metal inside the housing.

EnvEnt Ltd. also measured the incoming waste water and found it somewhat warm, but not obviously septic. Apparently the hydrogen sulphide was being generated in the primary tanks. The final effluent of the plant was however fully treated - the RBC system is remarkably robust and can accept a certain amount of incoming septic sewage.

EnvEnt Ltd. modelled the system and decided to employ the denitrification method in the primary tanks. We installed a temporary dosing system and eliminated the hydrogen sulphide in the first package plant within 24 hours. The dosing was then extended to the second plant and EnvEnt Ltd resolved the problem within two days.

We were next asked to study the remaining odour problems within the village. We modelled the entire sewage system and calculated the various potentials for sulphide production. There appeared to be only one possibility. We installed gaseous hydrogen sulphide data loggers where the waste water from the leisure complex (a pumped main that fed a gravity main over a long distance) entered the water service company's infrastructure. This confirmed that the effluent from the leisure complex was in fact septic on arrival, and producing a considerable amount of hydrogen sulphide in the gravity sewers of the village.

A test unit was installed dosing into the wet well at the leisure complex. Hydrogen sulphide in the air at the discharge point was logged. Within a few hours the problem was eliminated.

Following EnvEnt's recommendations, both sites were equipped with long term storage and dosing equipment, dosage regimes were optimised and the village has remained smell free ever since, thanks to our expertise in solving sewage smells permanently.