LIQUID V’S POWDERED LIME STABILISATION – OPERATIONAL EXPERIENCES TO ACHIEVE COMPLIANT SLUDGES FOR RECYCLING TO AGRICULTURAL LAND.
L. Keay, BSc (Hons), MSc, AMIChemE C. Dewhirst, BSc (Hons), MSc.
Wessex Water Services Limited
(free)
The Safe Sludge Matrix introduced treatment requirements for sludge processes to ensure that
potential pathogens such as E.coli 0157 and Salmonella spp are not transmitted into the food chain
through the use of sludge to fertilize food crops. As well as introducing two distinct standards of sludge
treatment, the matrix also introduced the principle of process control by means of Hazard Analysis and
Critical Control Points (HACCP) to monitor the treatment process, and assure the microbiological
quality of the treated sludge. Lime stabilization is currently being used by many water companies as it
is a relatively simple robust process that has a relatively low cost of ownership. Wessex Water are
currently operating three methods of lime stabilization: Feed forward lime dosing, back end lime dosing
and powder liming. Liquid lime is introduced to liquid sludge in a blending tank for a fixed residence
time before being dewatered through a belt press or centrifuge. Back end liming consists of liquid lime
being added to a dewatered cake. For the latter method powder lime is added to dewatered cake
through a screw feeder to achieve a mixed and compliant product. Samples for all methods are taken
to ensure compliance is achieved, however daily operating parameters are used to assess the
performance.
Wessex Water currently has 10 sites treating sewage sludge with lime stabilization; 4 using liquid lime
pre-treatment; 1 using back end lime treatment; 5 using powdered lime post-treatment. This paper
describes the operational experience from the 10 sites using lime stabilisation, and considers the key
operational parameters such as; grade of lime; lime dose rate; method of dosing; and type of mixing.
KEY WORDS
Dosing; HACCP; liquid lime ; powdered lime ; mixing; pathogen; pH
