Principal

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:

1. Coagulation sludge
2. Softening sludge

Coagulation sludge

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 hydroxide precipitates.

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.

Softening sludge

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:

• Discharge into sewer systems
• Thickening
• Conditioning
• Dewatering
• Coagulant or lime recovery
• Ultimate disposal

a. Discharge into sewer systems

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.

b. Thickening

Gravity thickening is used for both types of sludges. The following table illustrates the change in solids concentration:

Table 1 - Source: AWWA, 1969

Sludge type	Original solids concentration	Solids after gravity thickening	Thickener loadings, lb/day-ft2 (kg/day-m2)
Lime softening	1%	30%	12.5 (61)
Alum coagulation	1%	2%	4 (20)

c. Conditioning

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%.

d. Dewatering

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 :

e. Lime or Coagulant Recovery

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.

f. Ultimate disposal

The ultimate disposal of water treatment sludge entails two techniques:

1. Landfilling
2. Land application

   Landfilling

   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.

   Land application

   Alum sludge, at concentrations less than 25%, is best land applied. Sludges may be applied to croplands, to marginal 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: 6MB)

   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.

   Marginal land

   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.

   Forest Land

   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.

   Dedicated sites

   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 document below.

References

• Reynolds, Tom D. and Richards, Paul A. 1995, 2nd ed. Unit Operations and Processes in Environmnetal Engineering. PWS Publishing Company: Boston,
• AWWA. 1990. Water Quality and Treatment
• EPA

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