Management practices of water treatment sludge in Sri Lanka and re-use potential of sludge material

Abstract

Sludge remaining at water treatment plants is an inescapable byproduct of the water treatment process. The nature of sludge depends on suspended solids of raw water, coagulant type and chemicals that are used in the treatment process. Direct discharge of sludge into water bodies result in the risk of contamination of surface and ground water that affects water quality and aquatic biota. According to existing legislation, water treatment sludge is classified under industrial waste. Therefore, it is anticipated that the water treatment process would be legislated as a licensable activity in the near future. The National Water Supply and Drainage Board (NWSDB) which is the main potable water supplier in Sri Lanka, has paid attention to identify disposal routes, sustainable practices, and potential applications of water treatment sludge. The objective of this research was to recognize disposal practices and cost effective methods that conform to environmental regulations. To fulfill the objectives, a questionnaire survey was conducted pertaining to chemical usage, sludge production, sludge handling and disposal methods. To introduce sustainable practices, a series of experiments were conducted by adding sludge into production of burnt clay brick, replacing cement by sludge as an adhesive fine material in cement mortar and replacing sand by sludge as fine aggregate in Concrete Paving Blocks (CPB). The questionnaire survey revealed that 50% of selected treatment plants that are operated by NWSDB directly discharge the sludge into inland surface waters with no treatment or dispose to bare lands. Experimental results showed that the required compressive strength of burnt brick could be achieved by adding sludge up to 10% for load bearing walls of single storey buildings. Further, replacement of cement by sludge up to 30% in cement mortar, achieved the required flow of 105% to 115% with the water cement ratio between 0.7 and 1.1. Required compressive strength of cement mortar could be achieved with the addition of 10% sludge with the water cement ratios of 0.7, 0.9 & 1.1, 20% sludge with the water cement ratios of 0.7 & 0.9 and 30% of sludge with the water cement ratio of 0.7. The suitability of a CPB depends on its compliance to the compressive strength requirements. The results showed, the addition of 10% sludge as fine aggregate and 10% bottom ash and sludge as fine aggregate satisfies the requirement specified in the SLS standards for class 1. Hence Concrete Paving Blocks can be successfully produced using 10% of water treatment plant sludge as supplement for sand.Sludge production is an inevitable outcome of potable water treatmentand hence sustainable reuse techniques and disposal methods need to be introduced as a policy for protecting the environment.