Which Sewage Treatment Plant Is The BEST?

Basic Points

There are 8 essential things to find out when buying a sewage treatment plant, but these are the things that no-one thinks to consider.

Bent plastic sewage treatment tank

The warranty period for the sewage treatment plant tank itself. We have found that the quality of a lot of tanks has declined in the last few years. Many of the market leading manufacturers have reduced their tank warranties and several leading manufacturers have reduced it to only 12 months! Tanks do not crack or bend during the first 12 months after installation. If they are going to distort or crack, they do it when the ground has fully settled - 2 to 3 years after the tank was put into the ground. A 12 month warranty is of no use to you, but is of great value to the manufacturer who is not liable for any damage that occurs after the first 12 months.

The septic tanks above had only been in the ground for 2 years when they had to be replaced due to groundwater pressure distortion - even though the excavations had been backfilled with concrete. People forget that concrete is NOT waterproof. The water pressure builds up between the concrete and the tank itself and the tank must be strong enough to withstand this. There were around 50 collapsed tanks in this manufacturers yard!

Insist on a minimum 5 year tank warranty. If the tank hasn't cracked or bent after 5 years, it isn't going to.

No mechanical seals. Do not buy a sewage treatment plant whose tank is made in two halves and joined together using bolts, rubber seals, sealants, etc. If a tank has a seam it should be PLASTIC WELDED ONLY. The same applies to tank inlets and outlets - do not buy a plant that has sealing rings for the pipes to slide through!. Inlets and outlets must be spigots, welded or moulded as part of the tank. All seals eventually perish and fail and are impossible to replace, unless you dig the plant up again!

If a seal fails, then the plant is in serious trouble, as sewage can leak out and groundwater can leak in, flooding the plant way beyond its flow rate capacity. Raw, untreated sewage then flows out of the plant, blocking drainfields and polluting watercourses.

Electricity consumption of the sewage treatment plant. This varies enormously between makes of plant. For a 6 person sewage plant this can vary from 34 watts/hour to 583 watts/hour. Make sure that you know what size of electricity bill to expect.

Rotating disc sewage treatment plant

No internal moving parts. Avoid all plants that rely on internal moving parts - motors, gearboxes, discs, etc. as sewage is very corrosive and moving parts are made of metal - not a good combination for a long working life. They are also very expensive to replace.

Service and emptying intervals. These vary from 3 monthly to annual. Most sewage treatment plants require 6 monthly servicing, but some only require an annual service. A plant which needs an annual service saves around £130 per year. Beware of manufacturers who claim "Up to 3 years between emptying intervals". This is only true if they are very lightly used - "Up to" means nothing!

Will your sewage treatment plant be under-loaded? Most sewage treatment plants will only work efficiently down to 50% of their design size. However, minimum plant size is determined by the number of bedrooms you have. A 4 bedroom house requires a minimum 6 person plant, so if there is only 1 or 2 people living there, a 6 person plant is in trouble. If your plant will be under-loaded, buy an adjustable sewage treatment plant that can be turned up and down to suit the number of people using it.

Air Blower Position. Electric air blowers should be housed in a weather-proof kiosk OUTSIDE the tank. Some manufacturers try to reduce costs by housing them inside the tank, claiming that it makes fo a better appearance - BUT - inside the tank there is 100% humidity and the blowers suffer as a result. Also, in cold weather, water condenses on the cold lids of the tank and the water droplets drip onto the blowers and the shelf holding them, causing the blowers to fail. Never buy a sewage treatment plant that has the blower housed inside the tank. It may be a cheaper option for the manufacturer, but it will be expensive for you. Manufacturers put the blower in the tank as it saves them around £200 for the cost of a kiosk, but it is a stupid design option.

Treatment Performance This varies a great deal from one plant to another.

All sewage treatment plants must meet basic performance requirements for Suspended Solids, BOD and Ammoniacal Nitrogen reduction. They are performance tested for 38 weeks at an EN Test Centre for these pollutant reduction results. This allows them to obtain a BS EN12566-3 Certificate, which all treatment plants must have if they are for sale in the UK,(under the Environment Agency General Binding Rules) and Europe. However, some plants only just scraped through the testing for this Certificate, with average test results right on the limit for failure, whilst others sailed through it. You, obviously, want a treatment plant that has the best performance results

Click on the link for Official different sewage treatment plant performance results. Some sewage treatment plants perform 40 times better than others.

A 'Cheap' plant can cost you a fortune in the long-term.

If you can't find the answers to the above in the brochure, ASK the manufacturer the QUESTION.