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Mr M, Loddiswell

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Environmental Recycling Centre Ltd.

We have recently purchased a treatment works at Lee Mill where we can treat our waste in both an efficient and environmentally friendly manner. The post will also act as an additional location to keep our Lorries and tankers which will not only lessen the concentration of traffic in the Yelverton area but also allow us to operate more responsively and efficiently. Additionally the post will allow us to conserve fuel and save time and consequently we plan to extend further in the future.

Introduction

Sewage treatment is a multi-stage process to renovate wastewater before it re-enters a water stream or applied to the land. The goal is to reduce or remove organic matter and solids by using a dissolved air flotation (DAF) clarification system for the treatment and removal of suspended solids.

The plant is capable of processing a maximum throughput of 5 m3/hr. The treatment will consist of the addition of aluminium sulphate and sodium hydroxide to chemically precipitate the impurities and to control the process pH.

At present the consent to discharge is 300c.o.d

Preliminary treatment (Screens)

Untreated effluent from a tanker will discharge into the first section of the process which will screen out, or separate debris i.e. Sticks, hygiene products, large food particles, sand, gravel, toys, etc, are removed at this stage to protect the pumping and other equipment in the treatment process.

Primary Treatment

Primary treatment is the next step in the treatment process which separates suspended solids and greases from the wastewater. Waste-water is held in a quiet tank for several hours allowing the particles to settle to the bottom and the greases to float to the top.

Balance Tanks

From the primary tank it is fed into the balance tanks, which is fitted with a submersible mixer. The mixer minimises fat separation and ensures that a homogenous effluent is fed to the process plant. The balance tank is connected to the surge tank by overflow pipe. The surge tank is fitted with level probes that automatically transfers overflowed effluent back to the balance tank by operating the transfer pump.

DAF Dissolved Air Flotation Unit (Dependant on waste type)

The effluent from the balance tank is fed to the reaction tank at a controlled rate by the feed pump. The reaction tank is of a compartmented plug design and is fitted with an agitator in the first compartment. The process reagents aluminium sulphate and sodium hydroxide are pumped at a controlled rate into the first compartment and thoroughly mixed with the effluent by the agitator. The aluminium sulphate is dosed by pump at a proportional to flow. The sodium hydroxide is automatically dosed by pump at a rate to maintain the desired process pH. The process setpoint is set on the pH meter/controller.

The effluent flows by gravity to the second compartment where the resultant pH is measured by the probe from where it flows by gravity into the third compartment. On exit from the reaction tank, the flocculant solution is dozed at predetermined rate proportional to flow by the pump. Simultaneously a recycled stream of clarified effluent containing dissolved air is added. The impurities, air and flocculant combine to produce large buoyant flocs. These are fed to the DAF clarifier where the flocs quickly separate to form a layer of scum on the surface. A rotating scraper arm arrangement removes the scum from where it flows by gravity to a sump for transfer for further dewatering.

Clarified effluent is taken from the bottom of the clarifier from where it flows by gravity across a weir plate and discharged into the sump from where it is transferred to the biological tank.

Secondary Treatment (Biological Tank)

Secondary treatment is a biological treatment process to remove dissolved organic matter from wastewater. Sewage micro organisms are cultivated and added to the waste water.

The micro organisms absorb organic matter from sewage as their food supply.

Suspended systems stir and suspend micro organisms in wastewater. As the micro organisms absorb organic matter and nutrients from the wastewater they grow in size and number.

Final Settlement Tank

After the micro organisms have been suspended in the waste water for several hours, they are settled out as sludge. Some of the sludge is pumped back into the incoming wastewater to provide “seed” micro organisms. The remainder is wasted and sent on to the decant tanks.

Decant Tanks

Sludges are generated through the sewage treatment process. Primary sludges, material that settles out during primary treatment will require treatment prior to disposal. Secondary sludges are the extra micro organisms from the biological treatment process.

Untreated sludges are about 97% water. Settling the sludge and decanting off the separated liquid removes some of the water and reduces the sludge volume, decanted water will be returned back to the balance tanks.

Monitoring

Waste water treatment process requires careful management to ensure the correct operation, the management team at Glanville Environmental are committed to ensuring all it staff are trained and capable of monitoring the incoming sewage, the treatment process and the final effluent.

Glanville Accreditation UVDB Achilles Construction Line BSI CHAS