Sustainable management of water treatment sludge through 3‘R’ concept

Highlights

• Physicochemical characteristics of water treatment sludge are described.

• Beneficial reuses of water treatment sludge identified globally are discussed.

• Limitations and concerns over the success of various reuse options are highlighted.

• Further investigation and future issue requiring careful attention are discussed.

Abstract

The handling and disposal of voluminous sludge produced from water treatment plant poses difficulty to environmental scientists and engineers. In fact, its environmental friendly management and disposal is a challenging task and requires careful consideration. This article covers the physicochemical characteristics of water treatment sludge and its toxicity to environment and explores various options of constructive sludge application that may reform the handling of such waste sustainably. Several reuse options have been identified and investigated globally such as; coagulant recovery and reuse, as coagulant in wastewater treatment, as adsorbent for contaminants and heavy metals, as substrate in constructed wetlands, in co-conditioning and dewatering of sewage sludge, in cement production, in brick and ceramic making, in manufacturing artificial lightweight aggregate, as cementitious material and sand substitute in preparing concrete and mortar, in agriculture practice and in land based applications. However, the properties of water treatment sludge in majority depend on the chemical compositions that are important while deciding its potential reuse. It is also important to note that the sludge generated at different processing steps of a treatment plant varies in physicochemical characteristics. Therefore, an updated database on quantitative and qualitative analysis results of water treatment sludge produced along with their current disposal practices is needed to prepare sustainable application guidelines for various reuse options. Further extensive investigations are required to find a steady and reliable source of the sludge, with minimal compositional variability for fearless and optimum utilization. Present review is focused on the beneficial reuse in various environmental settings. Wide and variable sludge utilization methods have been reviewed in depth with associated pros and cons. This document may help in developing some ideas of suitable sludge management strategies for sustainable development under stringent environmental norms.

Introduction

The trend of urbanization and rapid growth of population have exerted the portable water demand, which requires exploration of raw water sources, developing treatment and distribution systems. In order to meet the increasing demand, the production of potable water from the water treatment plants (WTPs) has to be increased. Surface water mainly from river carries suspended as well as colloidal solids and other impurities. Therefore, requires proper treatment, depending on the quality of available raw water and quality needed at the users end. The conventional WTPs involve the process of coagulation, flocculation, sedimentation, filtration and disinfection. During these treatment processes large quantity residues or wastes are generated known as water treatment sludge (WTS) and a typical WTP produces about 100,000 ton/year of sludge whereas, on a global scale, available literature estimates that at present the daily production of sludge exceeds 10,000 ton (Babatunde and Zhao, 2007). The WTS is of environmental concern and requires careful consideration if it is to be managed in an environmentally acceptable and sustainable manner.

In India, WTS from most of the WTPs are being discharged into nearby drains, which ultimately meet the water source on downstream side of intake. In some of the WTPs, clarifiers are cleaned once in a year and the sludge is disposed off on nearby open lands (CPCB report, 2011). The simple method of final disposal, although a less expensive, is not a proper solution due to the possibility of contamination of water bodies and soil from the chemical products used in the treatment. Proper handling of increasing residual sludge from WTPs in an economical and environmentally friendly manner remains a very important issue. The recovery, recycling and reuse, 3‘R’ may provide a sustainable solution to the WTS management problems under stringent environmental laws. Therefore, the main objective of this paper is to present a comprehensive review of sustainable approaches and prospective trends in sludge handling that contributes to the WTS reuse.