Whilst the R.B.C. is the sewage treatment plant of choice, there are two other distinct types that I.H.S. Systems Ltd. install. There are a few sites where the R.B.C is unsuitable, such as those prone to flooding, so these are a fair alternative, although they do have a few problems.

The first is the S.A.M. unit. This is short for Submerged Aerated Media.  The plant comprises a tank filled with pieces of plastic (the media) to which bacteria adhere. The plastic is then submerged in liquid sewage which provides the food element for the bacteria. The oxygen for the bacteria is provided by a blower or compressor blowing bubbles through the liquid sewage.

It is a simple process, but is ''hit-or-miss'' compared to the managed flow R.B.C. Sudden large influxes of sewage from baths or washing machines can displace liquid before it is properly treated by the bacteria. This is compensated for to a degree by providing a large primary settlement stage.

Other problems with this type of installation relate to noise from the compressor or blower; especially after a few years use. Occasional odour is noted if large sewage influxes displace untreated sewage. Sludge build-up in inaccessible places can reduce the efficiency of the plant.

The second is the trickling filter. As the name implies, liquid sewage is trickled through a bed of plastic media (the filter) in which the bacteria live. As the sewage passes through the bed, nutrients in the sewage are consumed by the natural bacteria colonising the filter. The partially treated liquid is collected in a separate tank beneath the bed, and pumped back to the top of the plastic media to repeat its treatment. Finally it is displaced by incoming sewage and passes out into a surface watercourse. In common with the S.A.M. units, sudden large influxes of sewage can displace sewage before it is properly treated. The pump used for these units  has a normal life of 3 to 5 years, but is easily replaced.

Overall, the final treated effluent from these units generally complies with the purity requirements of the Environment Agency, but it is essential to note the building regulations requires that any new unit purchased must carry a CE mark otherwise it will be rejected by the building control officer.