by Murad Pandit and Siddharth Das
What is sludge?
Water treatment sludge is defined as 'the accumulated solids or
precipitate removed from a sedimentation basin, settling tank, or clarifier in a
water treatment plant'. The accumulated solids are the result of chemical
coagulation, flocculation, and sedimentation of raw water. There are two types
of water treatment sludges:
These sludges have a gelatinous appearance are produced from clarifier
operations and from the backwashing of filters. They contain high concentrations
of aluminum or iron salts with a mixture of organic and inorganic materials and
Dewatering of coagulation sludges is a difficult task and in the past the
sludges were discharged into a water source, like a river or a lake. However,
nowadays the sludge is processed for ultimate disposal and backwash and
clarifier water is returned to the treatment facility for reprocessing.
These sludges contain mainly calcium carbonate and magnesium hydroxide
precipitates with some organic and inorganic substances. These sludges dewater
easily and processing for ultimate disposal is common and feasible.
The unit operations and processes for sludge handling in water treatment
facilities areas follows:
This technique of disposal is used mainly for coagulation sludge, which can
be processed by a waste treatment facility with additional capacity. Softening
sludges cannot be disposed in this manner because they have a higher volume and
encrust weir, channels, digesters, piping, etc.
Gravity thickening is used for both types of sludges. The following table
illustrates the change in solids concentration:
Table 1 - Source: AWWA, 1969
||Original solids concentration
||Solids after gravity thickening
||Thickener loadings, lb/day-ft2 (kg/day-m2)|
To achieve better dewatering results, the coagulant sludge may be conditioned
through heating in reactors or by freezing and thawing, which causes the bounded
water to be released due to extreme temperature and pressure conditions. Using
heat treatment and the freezing and thawing techniques, the solids concentration
can go up to 20%.
The results from dewatering are different for the two sludges since the
coagulant sludge is harder to dewater than the softening sludge. The following
table shows a range of solids concentrations for different dewatering techniques
Table 2 - Source: Reynolds & Richards, 1995
||Rotary vacuum filter
Lime sludge from water softening may be separated into calcium carbonate and
magnesium hydroxide by centrifuges through calcification. Lime is recalcined to
produce reusable quicklime. Alum recovery is not very commonly used but is
accomplished through acidification with sulfuric acid.
The ultimate disposal of water treatment sludge entails two techniques:
Landfills may be on public land such as a municipality owned landfill, or
on private land. Landfill operators commonly require 15 to 30 % sludge
(solids). The minimum concentration required is often determined by local
sanitary landfill regulations.
For alum sludges, (the most common in the US drinking water plants)
effective landfilling requires the solids concentration to be at least 25%. At
lower concentrations, land application is more appropriate.
Alum sludge, at concentrations less than 25%, is best land applied. Sludges
may be applied to croplands,
land for land reclamation , to forest
land or to dedicated
sites. Other than at dedicated sites, usually no more than 20 dry tonnes
of sludge per acre is land applied.
Cropland (see video:
Sludges are applied to cropland either by surface spreading, or by
subsurface injection. Surface irrigation methods include specially
equipped farm tractors, trucks or special applicator vehicles. Sludge is
usually applied once a year to a given area.
Sludge has been applied to marginal land for recalmation in Pennsylvania
and in other states successfully. This is usually a one-time process and a
continual supply of land must be provided for future applications.
Application to forest land has been done successfully in Michigan,
Washington and South Carolina. It is determined by sludge characteristics,
tree maturity, species, soil etc. Application to a specific site is often done
only at multi-year intervals.
Since such sites are exclusively used for land application, the application
rates are much higher than in the other means discussed above, ranging from 20
to 200 dry tonnes of solids per acre of land. Sludge can applied to dedicated
sites throughout the year.
Costs of ultimate disposal of sludge
Costs can be an important concern in waste disposal and often play an
important part in determining the disposal method used. This program computes
Capital and O&M costs for landfilling at a municipal landfill and for land
application to farmlands. For landfilling, the input requires the annual
volume (million gallons) of sludge to be landfilled on a 20% solids basis. For
land application, two inputs are required. The annual volume of 5% sludge to
be land applied and the number of days per year for which land application is
performed. The estimates are very tentative and are only illustrative. For
detailed information see EPA
- Reynolds, Tom D. and Richards, Paul A. 1995, 2nd ed. Unit Operations
and Processes in Environmnetal Engineering. PWS Publishing Company:
- AWWA. 1990. Water Quality and
Send comments or suggestions to:
Student Authors: Siddharth Das email@example.com, Murad Pandit firstname.lastname@example.org
Faculty Advisor: Daniel
Copyright © 1996
Last Modified: 02/24/1998