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

1. The removal of polluting matter from the sewage flow as solids or slurries of solids in water (sludges)
2. The removal of polluting matter from the sewage flow and separated sludges by accelerated natural processes of biochemical breakdown brought about by microorganisms
3. The separation of water from sludges to reduce the volume of sludge for disposal

Often recognised as four major stages in the treatment of sewage

• Preliminary treatment (screening)
• Primary treatment
• Secondary Treatment
• Tertiary Treatment

However, in detail the processes occur during the six stages in the treatment of sewage for disposal

1. A preliminary screening to remove grit and solids
2. A primary settlement stage in tanks to allow the removal of solids and grease
3. Secondary treatment involving the microbial oxidation of organic matter and NH₃ and the further removal of solids
4. A further part of secondary treatment is often referred to as a 'polishing' treatment using sand filtration to remove very fine solids from the effluent before discharge.
5. Tertiary treatment under anaerobic conditions to bring about denitrification to remove N and chemical precipitation to remove P
6. A sludge treatment stage involving combinations of digestion, thickening, dewatering and drying to prepare the sludge for disposal. (also produces methane which can be used for power and heat generation)

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.

Sludge Composition

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:

• Metals
• Organics
• Disinfection

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 & Prevention Control Regulations (IPPC), 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:

• Non food-chain outlet
• No time constraints through year
• Coniferous trees can utilise nutrients throughout year
• Sites are remote - avoid nuisances
• High grade roads for timber extraction - used for heavy sludge vehicles

Constraints

• Within rotation (50years) periods when not available
• Terrain can be more difficult than agricultural locations
• Public access
• The disposal of sludge to forestry sites is not yet subject to EU control

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 (2^nd Edition) or Mason, 1996 (3^rd 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.

Click here to return to the BIOL202 Homepage

This page is maintained by an 'amateur webmaster': Dr Rick Leah, Jones Building, School of Biological Sciences
Any feed back or comment would be welcome, but please be gentle! 
NB: The HTML has not yet been 'verified' - apologies to any disabled users - I will try to attend to this during this academic year