Technical

Sewage sludge treatment

Sewage Sludge Treatment

Biosolids treatment by Lime Stabilisation can be achieved by applying a controlled dose of liquid lime, hydrated lime or quicklime to sewage sludge. This is considered the best method of treatment to produce a valuable end product for land based recycling, both simply and efficiently.

Guidelines agreed between ADAS, Water UK and the Retail Consortium – the ”ADAS Safe Sludge Matrix” effectively banned the use of untreated sewage sludge on agricultural land from 31st December 1999. Under these guidelines, sludge must be treated to either conventional or advanced treated standards before recycling to agriculture. Conventionally or enhanced treated standards refer to the level of pathogen destruction in the end product. Lime stabilisation is easily capable of achieving both of these standards.

The draft EU regulations encompass these principles for sludge treatment and terms treatment standards as conventional and advanced treatment. “Advanced Treatment” is a European Union designated term for the level of treatment which sanitises sludge by reducing the number of micro organisms to a negligible level.

Hydrated lime or liquid lime can be added simply to raise pH, whereas quicklime will give a rise in pH and temperature and reduction in moisture content. These methods can be used to produce a Biosolids end product suitable to be recycled to land as a valuable fertiliser and soil conditioner.

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.

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 it’s 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(1), this dries and pasteurises the sludge. This temperature regime will produce an advanced treated sludge as recognised by the draft EU regulations.

Typical quicklime addition for advanced treatment by temperature rise

Addition of 50-90%* CaO per unit of dry solids (DS) gives an advanced treated sludge at >55 °C and pH >12 for 75 minutes. * depending on DS contents

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.

Addition of 20-40%** CaO or equivalent quantity of Ca(OH)2 per unit of dry solids (DS) is capable of producing a treated sludge. ** depending on sludge buffering capacity

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 be dosed as a liquid lime suspension prior to dewatering.

Process Advice

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

Advantages of Lime Treatment

• achieves up to 6 log reduction of pathogens
• stabilises sludge with no risk of pathogen regrowth
• eliminates odour, reduces vector attraction
• low investment costs, small area required for plant
• simple to run, easily automated, 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, enhances growth environment in the soil
• maintains nutrients bio availability
• Advanced Treated Sludge product is a “soil like” pathogen free soil conditioner which remains stable in storage for extended periods. It is friable, easy to handle and spread making it a first choice product for agricultural and other applications.

(1) Draft EU regulations.