School of Biological Sciences
Natural Resources: depletion, pollution and toxicology
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Natural Resources: depletion, pollution and toxicology[ BIOL202 Home | Aquabiology at Liverpool ] |
Sewage Sludge a Human & Environmental Dilemma
Additional info for a Lecture given by Dr Rick Leah
Introduction
Sewage treatment is now achieved in a reliable and safe manner in most developed countries. However, despite bacterial and fungal oxidation of most of the constituents of sewage, there is an irreducible amount of organic-rich sludge to be disposed of. This is a large-scale problem that grows in magnitude with our increasingly affluent lifestyles. Total sludge production in UK in 1989 was around 1.2 million tonnes dry solids (Taylor & Moffat, 1989). It will be considerably higher now and has been predicted to rise to around 2.2 million tonnes by 2006. Pressures to improve environmental protection have forced the disposal of sludge to sea to cease in December 1998. However, the production of sludge carries on relentlessly so that new methods for disposal had to be found. This has been achieved, at a cost. The dilemma is that this cost has to be set against healthcare, improved roads etc.
The Sewage Treatment Process
Three basic processes
Often recognised as four major stages in the treatment of sewage
However, in detail the processes occur during the six stages in the treatment of sewage for disposal
Composition of Sewage
Sewage actually contains only small amounts of polluting
matter on a volume basis
Domestic sewage contains ca 0.1% of impurities in water (equiv to
1000 mg l-1)
70% of impurities are organic
include proteins, urea, sugars, starches and cellulose, soap
detergents, cooking oil and greases
The inorganics include chlorides, metallic salts and road grit where storm water is included
Raw sewage
ca 400 mg l-1 suspended solids
ca 300 mg l-1 of BOD
Final effluent from well operated STW
< 15 mg l-1 suspended solids
<=15 mg l-1 of BOD(5day)
<10 mg l-1 of NH3
Sewage Treatment Processes
Septic Tanks
Often used for very small communities or single houses in rural locations
Trickling Filters
In the UK, now largely restricted to modest sized, rural communities where land use is not very contentious.
Activated Sludge
A highly industrialised process favoured for sewage treatment for large urban areas. It is capital intensive but efficient in terms of land requirements. It 'scales' well as populations increase in size.
Whatever the circumstances there are pressures on Utility companies to recycle as much material as possible. This can be achieved by disposal on Agricultural land because of the 'beneficial use' of the nutrient content.
In terms of dryweight etc 
In terms of Heavy Metals
Disposal Routes for Sludge
Sludge poses various problems for its safe and economic disposal. It has a very high water content (usually >95%) and the sludge is technically difficult to dewater. It does contain some useful plant nutrients.
Contaminant Problems etc
Wherever sludge is disposed there are some problems:
It takes a significant time for bacteria and viruses to decline within the sludge so that various restrictions are imposed on the use and disposal of sludge (again covered by EU legislation)
Now that the quantities of toxic substances in industrial discharges have been controlled at source under the Integrated Pollution Control Regulations (IPC), the amount of Nitrogen in the sludge usually becomes limiting for the disposal to agricultural land before metals & other toxics.
Disposal Routes to Land:
1 Agriculture
Constraints - EC Directive (1986) on protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture.
It imposes Limit values for the annual addition of metals to agricultural land
The sludge can be delivered free - the acceptable distances vary with the Utility Company involved from 1-2km to 55-60km (Costs ca. 10p /km/tonne)
450,000 tonnes (on a tds basis) of sewage sludge currently applied to agricultural land in the UK (Taylor & Moffat, 1989) ie around 40% of production)
2 Land reclamation
To maintain an adequate supply of nitrogen, a capital of over
1,000 kg N/ha is required in soil
The actual requirement ca. 100 kg /ha /yr - provided by
mineralisation of ca. 1/16 of what is present
3 Forestry
It is estimated that there are 2 million ha of productive woodland in UK
Attractions:
Constraints
There is some concern over the mobility of metals in forest soils because of the frequency of acid soil conditions under trees.
It is estimated that up to 11% of the total sludge production of 1.2 million dry solids could be applied to forest soils in the UK (assuming transport distances to 32km (reduced to 6% if distance reduced to 16km)
A research programme carried out in this university concluded that sewage sludge can significantly benefit tree growth on podzolic, ironpan and restored soils for spruce and pine plantations (Byrom, 1984)
4 CoDisposal with other wastes to landfill
In the UK disposal of sludge to landfill (80,000 tds) accounts
for about 10% of sludge disposed to land.
Addition of sewage sludge to the mixtures in landfills changes
conditions in tip - gas generation, leachate quality.
increases gas generation - useful where this is collected.
Improves rate of stabilisation of wastes.
Disposal Routes to Water: Ocean Dumping
300,000 tds (Taylor & Moffat, 1989) was dumped at sea by the UK before the implemtation of the 1998 Directive (approx. 30% of annual production). This was brought to an end by the EU Directive on Sludge Dumping at Sea
Case study - Merseyside
Out of Sight, Out of Mind
Two large conurbations - Manchester and Liverpool
Different circumstances for each of them - river and estuarine locations
Victorian provision in Manchester
Large sewage works at Davyhulme
Sludge transported down Manchester Ship Canal - dumped in Liverpool Bay
Problems - accumulation of persistent pollutants in the marine food chain
There was no provision for sewage treatment for most of the population living near the Mersey estuary in Liverpool and Birkenhead before 1980. This was because the fast currents in the estuary moved most of the problems away from contact with people.
1980s - improvements needed for Mersey Estuary - comprehensive system installed
Collector sewer to Sandon Dock
Replicated on Wirral
A Connector Pipeline was constructed from the Manchester are to Sandon Docks, a redeveloped dock within the Liverpool dock system. This eliminates the long, slow ship journey along the Manchester Ship canal to minimise the costs.
Eventually the pipeline was extended around northern Manchester and collects together the sludge production of a large number of sewage works.
Unfortunately for the North West, there was a change in Regulations governing Dumping at Sea almost as soon as the new facility was completed - a new solution needed
Eventually, the Mersey Valley sludge processing facility at Shell Green near Fiddlers Ferry was developed.
It takes sludge from approx 40% of the population of the North West (an area which extends from Carlisle to Crewe)
ca 2million wet tonnes per annum are handled at approximately 3% dry solids
It is converted into ca 250,000 tonnes of sludge cake (currently approx 27% dry solids)
The majority of the sludge is disposed of to agriculture which is the cheapest option
Some is disposed of to Land Reclamation
The remainder, some 70000 tonnes has to be incinerated
Sludge Processing
Water squeezed out - plate presses 9 of them at Shell Green
The process converts solids from ca 3% on a volume basis to 27% of dry solids
The liquor squeezed out of the sludge is put back into the public sewer for treatment via Liverpool STW.
Various flocculants - Ferric Chloride and polyelectrolyte have to be added to the liquid sludges to change the particle properties to assist filtration.
Incineration
This has to be a very carefully controlled process, partly because of the adverse public reaction to the environmental deficiencies of past incinerators.
As a consequence, when the Shell Green plant was being planned, there had to be a degree of 'Design Overkill' to ensure success in planning process. This knocks-on today in increased running costs fro processes which are not really needed to meet design emissions criteria.
Incineration in modern fluidised bed incinerators - becomes autothermic for sewage sludges somewhere just over 30% dry solids. The process is initiated (and any deficiency made-up) with Natural gas.
The fluidised bed of the incinerator consists of sand + small amount of limestone to eliminate sulphur emissions
The effluent gases from the furnace go through three stages of filtration to remove particles which are disposed of with the ash.
The gases are cooled for 'Wet scrubbing' with acid and alkali and then heated up again so that the final plume rises up into the atmosphere without falling back in close proximity to the plant.
The effluent from the scrubbers needs treatment : neutralisation, & settlement
This is done in a small on-site treatment plant producing a chemical sludge. This sludge is sent directly to landfill since it has little or no organic content.
Residues
Even after incineration, there are ca 7 - 10,000 tonnes of ash produced per annum by the plant (because ca. 40% of dry solids are left as ash)
Most of the ash is sent to landfill where it has some beneficial use if it is mixed with various wet wastes to make them easier to handle.
There is some experimental use of the ash as an additive to cement products
Costs
The Sludge Processing Facility at Shell Green cost approx. £100 million to build (at 1998 prices).
It now costs around £7 million a year to run with ca 7 operators per shift plus a modest number of other staff on day work.
Power Generation
The average power generation in 2000 was:
377,000 Kw hr /month
4.5m Kwh per annum
The process is currently being optimised - 800 Kw hr /month was achieved in February 2001 - making it almost self-sufficient in power terms.
Further reading
Sewage sludge production and chemistry are covered in a number of more general texts (eg. Alloway and Ayres, 1997 (2nd Edition) or Mason, 1996 (3rd Edition)) but for a comprehensive (if slightly dated) account see Hall 1992.
Refs
Bradshaw, A.D., Southwood, R. & Warner, F. (eds.) (1992) The Treatment and Handling of Wastes Chapman and Hall for the Royal Society 302 pp (628.34 B81 (Eng))
Byrom , K.L. 1984 The use of sewage sludge in land reclamation. Univ of Liverpool, PhD Thesis.
Commission of the European Community (1986). Council Directive on protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture. Off J of the Europ. Comms., L18/6-12
Hall, J.E. 1992 Treatment and use of sewage sludge. pp 63 - 82 (In Bradshaw et.al, Eds)
Mason, Christopher F (1996) Biology of freshwater pollution Publisher: Longmans Scientific & Technical, Harlow, Edition: 3rd ed.