Waste water treatment centre, Cambridge

Waste Water Treatment Centre Cambridge 

Thirty-three biologists braved the elements and aromas on an early evening in late September to discover what becomes of the raw sewage from the City of Cambridge.

The first Cambridge sewage farm in 1895 consisted of a 69 acres site which expanded to 102 acres the following year, with the treated effluent entering the River Cam at Baits Bite Lock.  The land was also utilised for crops, with a total value of crops and assets of £1430 in 1916.  Further land was acquired over the years so that by 1937, new works were constructed to manage the sewage from a growing population.  Filtration beds built at this time are still in use today, the basic biological technology unchanged. In 1994 a 6 km deep tunnel was commissioned to transfer city sewage to the treatment works at Milton on the edge of the city. 

Today the Sewage Treatment Works at Cambridge serves approximately 165,000 people, with average volume treatment of 26 million litres at day – accounting for some 5.2 million toilet flushes, 650 thousand 5 minute showers and 325 thousand baths

A three metre diameter pipe brings effluent directly from the city at 200 litres per second.  Firstly there is an inlet screen for the removal of ‘gross solids’ such as paper, rags, plastic bottles and cans, all of which is transferred directly to a skip. Following this is the removal of grit and sand, which is washed off the roads and drive ways. The resulting screened sewage is then gravity fed to five cornet shaped primary settlement tanks where 70% of the solids and 40% of the BOD (Biological oxygen demand) are removed over a 6 hour period. 

The main part of the process is the secondary treatment, where ammonia and BOD is further removed from the sewage, which is carried out by Biological filtration in granite media filtration tanks.   Final settlement tanks, removes remaining solids from this secondary treatment which are carried off, together with solids collected from the tertiary treatment.  The resulting sludge is thickened by gravity thickeners reducing the water content, then an enhanced enzymic digestion step, followed by pasteurisation at 55^oC for several hours, producing methane gas which is used on site as an energy source.  Finally the digested sludge is centrifuged to dewater and a ‘cake’ is produced that is used in agriculture as fertiliser.  This fertiliser can only be used on DEFRA approved sites, where crops grown are not the variety which are eaten raw. 

Effluent released into the River Cam today is constantly monitored to ensure tight standards are maintained and odour control units have been successfully implemented to the delight of the local community. 

Liz Campbell