Technical

Sewage sludge treatment

Benefits of using lime for sewage sludge treatment

Lime benefits the treatment of sewage sludge as it:

• Is a registered biocide
• Achieves up to 6 log reduction of pathogens
• Stabilises sludge with no risk of pathogen regrowth
• Eliminates odour, reduces vector attraction
• Has low investment costs, only a small area required for plant
• Is simple to run, easily automated, and is available as mobile plant
• Converts sewage sludge into a biosolid product
• Increases the dry solids content, improves structure, handling properties and spreadability
• Adds calcium hydroxide and organic matter to improve the soil structure, enhancing growth environment in the soil
• Maintains nutrients bio-availability

Introduction to sewage sludge treatment using lime

Biosolids can be sanitised by adding a controlled dose of hydrated lime (calcium hydroxide) as a liquid or quicklime (calcium oxide) as a dry powder to sewage sludge. Hydrated lime as liquid lime can be added simply to raise the pH. Calcium oxide addition will give a rise in pH, temperature and reduction in moisture content.

Lime is considered the best method of treatment to produce a valuable product suitable to be recycled to land, as a fertiliser and/or soil conditioner with enhanced calcium content, a major crop nutrient. The sludge will also contain high levels of phosphate, from washing powders and detergents, meaning less phosphate has to be mined and transported.

Calcining to make calcium oxide emits carbon dioxide This is partly negated when the oxide reacts in sewage sludge to form hydrate, which then absorbs atmospheric carbon dioxide to return back to carbonate. Taking a wider view shows further CO₂ reduction. By generating a useful fertiliser, the crop yield is increased, meaning a greater CO₂ uptake by the crops per hectare and greater crop yield per hectare reduces the operational carbon footprint. Having an amount of phosphate in the sludge means less virgin phosphate has to be mined and transported, further reducing the amount of CO₂ that would be emitted.

Lime stabilisation is noted as an example of effective sludge treatment processes in the "Sewage sludge in agriculture: code of practice for England, Wales and Northern Ireland" (available here) and as a common treatment method, which can have the benefit of neutralising soil acidity, in the "Scottish Environmental Protection Agency (SEPA) Guidance on the suitable organic material applications for land restoration and improvement" (available here).

The mechanisms of pathogen reduction

1. Raise temperature (quicklime addition only)

Once quicklime is mixed with the moisture bearing sludge an exothermic (heat producing) reaction takes place.
CaO                 +          H₂O      =          Ca(OH)₂           +          heat
quicklime                    water              hydrated lime             1,140kJ/kg CaO

In a homogeneous mixture, the quicklime reacts with the moisture present in the sludge. This exothermic reaction generates significant amounts of heat energy which will dry the sludge (temperatures can reach in excess of 100ºC) as well as chemically binding 32% of its own weight of water as hydroxide.

If the correct quicklime dose is applied, the temperature will be easily maintained at over 55ºC for 75 minutes – which complies with draft EU Regulations. This dries and pasteurises the sludge. This temperature regime will produce an advanced treated sludge as recognised by the draft EU Regulations.

Additions of between 50% and 90% CaO per unit of dry solids (DS) gives an advanced treated sludge at >55°C and pH >12 for 75 minutes. The required addition level to achieve advanced treatment depends on dry solids content.

This is a recognised method of producing an advanced treated sludge.

2. Raise pH (quicklime, hydrated lime or liquid lime addition)

If the correct quicklime, hydrated lime or liquid lime dose is applied, the treated sludge will be elevated to pH 12 for at least 72 hours.

Depending on the sludge buffering capacity, between 20 and 40% CaO per unit of dry solids (DS) - or equivalent quantity of Ca(OH)2 or liquid lime - is capable of producing a treated sludge.

The process

Quicklime addition

Quicklime is added to a dewatered sludge cake and thoroughly mixed (see example below).

Hydrated lime or liquid lime addition

Hydrated lime, or liquid lime, is typically dosed as a liquid lime suspension prior to dewatering.

Process advice

British Lime Association members are able to advise on process design and optimisation.

Safe handling

The Safe Handling of Lime document (available here) gives some general guidelines on the handling requirements for lime products.

However, please refer to the supplier's Safety Data Sheets for the complete safety information referring to an individual product being considered.