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SLUDGE TYPES
The volume of liquid sludge produced at a sewage treatment works usually represents approximately 1-2% of the total flow of sewage, but treatment and disposal can account for approximately 30-50% of the running costs of the works. Sludges from conventional sewage treatment plants are derived from primary, secondary and tertiary treatment processes.
- Primary Sludge
Consisting largely of faecal solids and will also contain paper, sanitary and medical products, kitchen wastes, grit and other mineral matter. Pre-treatment, i.e., inlet screening, grit removal trap, etc., will remove the vast majority of non-biodegradable material and should always be used whenever possible. Primary sludge will also contain a variety of pathogenic micro-organisms. A typical domestic primary sludge will contain approximately 5.0-6.0% dry solids and does not normally require thickening prior to further treatment.
- Humus Sludge
Humus sludge is the product of settlement of effluent from biological filters, submerged aerated filters, etc. and is mainly bacterial and fungal material sloughed from the filter media. A typical humus sludge contains 0.5-2.0% dry solids and is not easily thickened/dewatered alone.
- Surplus Activated Sludge
In an activated sludge plant, polluting matter is transferred to the mixed liquor suspended solids thus increasing its mass. To maintain an optimum level of mixed liquor suspended solids a portion of sludge is removed at regular intervals and is known as surplus activated sludge. The sludge will consist mainly of flocculated and synthesised solids and micro-organisms. Surplus activated sludge rarely contains more than 1.0% dry solids and requires thickening prior to further treatment.
- Tertiary Sludge
Tertiary sludge is derived from tertiary treatment processes and comprises the fraction of secondary sludge that remains in the effluent from the secondary clarifier that is removed by the tertiary treatment filters. Tertiary sludge forms a very small part of the total sludge production of a works and is normally returned to the head of the works for further treatment or co-settlement. Sludge produced during physico-chemical treatment for nutrient removal from secondary effluents, i.e., phosphorous precipitation, is included in this category. Tertiary sludge rarely contains more than 1.0% dry solids.
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