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The A to B of sewage conveyance

Starting with sewers

Sewers are underground pipes that enable our wastewater to flow, initially by gravity, from our houses, offices and factories to the Wastewater Treatment Works (WwTW). Most sewage is water borne, which means that water is the transporting medium for moving solid wastes (the pooh from your loo and small scraps from your sink) and associated dirty water along a pipe-work drainage sewer network.

On your marks, get set, go!

Typically, a sink, bath or loo-pan full of sewage may travel many kilometres before it reaches the processing facility or WwTW. Depending on several factors, this journey can take up to 96 hours or more, especially in sprawling seaside towns with 'migrant', seasonal populations.

Although, the initial and subsequent propulsion is by gravity (water flows downhill), moving sewage over flat ground or up over a small hill, which is often the case between a suburb and the municipal WwTW, devices called lift stations and pump stations are used. These installations are like energy nodes that pass parcels of effluent along trunk sewers of increasing diameter (to accommodate tributary sewers) and ultimately to the WwTW.

Pump stations
There are three types:

  1. Lift stations are commonly found after the Inlet Works within a WwTW where their function is to lift the effluent so it will move through the works by gravity. Generally they employ a pair of inclined open-topped Archimedean screws fitted snugly into an open-topped, semi-cylindrical channel. As they contra rotate, they elevate both liquids and suspended solids (the screws exerting a macerating effect on floaters) from the feed boot for a short distance upwards and discharge into a primary settling tank. Lift stations have relatively high maintenance costs and their propensity to release odour have lead to their replacement by 'pump stations', which use closed pipes.
  2. Pump stations are what they imply. A pair (or more usually) of pumps, controlled by automatic float level or laser switches, placed in the bottom of a 'wet-well' that fill, over time, from the input sewer line and share the evacuation cycles of effluent. The discharge side is called a 'rising main' and it empties into a 'trunk sewer' – an inclined gravity sewer that runs just below the ground surface level and discharges down-line into the next pump station. A pump station may have multiple inlets, hence the need for progressively increasing pipe diameters and more powerful pumps as the sewer progresses towards the WwTW.
  3. Transfer stations are pump stations that push effluent along flat or rising ground. The 'head' or height the effluent is pumped depends upon the strength of the pumps employed.

Common sewage conveyance problems

  1. Pump stations and their equipment are in constant threat of malfunction caused by excessive amounts of fats, oils and greases (FOG) in the modern day effluent they move along the sewers. Deposits of FOG build up on the walls, pump suspension chains and cables, and often clog the automatic float switchgear. Ultimately a 'fat raft' forms over the surface, which can lead to float switch failure and eventually overflows.
  2. Odour is a common problem in pump stations. Especially with slow cycling stations and long delivery rising mains in excess of 500m, there is a danger that the sewage in the line will ferment during rest periods. It's important to note that the non-return valves will always ensure the rising main is full. The latent microbial activity within the sewage, little though it may sometimes be, is often sufficient to deplete any oxygen dissolved in the effluent. This permits anaerobic bacteria to become dominant and the production of sulphide gasses occurs. Offensive odour and corrosion results.
  3. Public reaction. Odour is always a real problem for the municipal engineer. Spilled effluent may pass unnoticed, but bad smells incite the public to complain to the authorities.

The BIO-SYSTEMS solution

BIO-SYSTEMS SA has a selection of products that are very effective in remedying problems associated with sewage conveyance.

  3. BIO-SYSTEMS L2120
  4. BIO-SYSTEMS Drain Flow 60 (DF60)
  5. BIO-SYSTEMS Hard Surface Degreaser (HSDG)

The reason these products work so well

BIO-SYSTEMS products break down COD and fat deposits into their natural constituents. They help counter the build up of slimes and the generation of sulphide gas, which forms sulphuric acid, causing corrosion to sewer pipes, brickwork, inspection chambers, metal fittings and electric panels in pump stations. There are no deleterious deposits. The contaminants are biologically degraded to natural substances. There is NO RESIDUE.

Sewage conveyance system  Sewage conveyance system before BIO-SYSTEMS treatment

Sewer conveyance system after BS product  Sewage conveyance system after BIO-SYSTEMS treatment

Sewers transport wastewater from your home to the WwTW. Go to WwTW for more on the clean-up process.

For a complete list of BIO-SYSTEMS products and their target pollutants click on our Product table. Also visit our Case histories.

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