A cost analysis of sewage sludge composting for small and mid-scale municipal wastewater treatment plants

Abstract

The costs of building and operating windrow, aerated static pile (ASP), and horizontal agitated solids bed (HASB) sewage sludge composting facilities for small and mid-scale municipal wastewater treatment plants are analyzed. The capital costs and total annual costs of the composting facilities analyzed range from US$164 200 to US$349 000 and from US$100 000 to US$295 700, respectively. On a per-input-dry-Mg basis, composting costs range from US$55.31 to US$173.66/dry Mg handled depending on the type of system and the moisture content of sewage sludge. Moisture content of sewage sludge is the key factor of capital costs and operating costs for different composting facilities. The lower the moisture content of sewage sludge, the less the capital and operating costs for different composting facilities. A windrow composting facility is cheaper to build and operate than ASP and HASB composting facilities. The dominant components of capital costs for windrow are land acquisition and surfacing, and that for ASP and HASB is equipment. Bulking agents and labor are the key factors of operating costs for sludge composting facilities. The availability of land and the costs of bulking agent will significantly limit the spreading of windrow system in small and mid-scale municipal wastewater treatment plants; however, it is suitable for spreading ASP and HASB sewage sludge composting systems for small and mid-scale municipal wastewater treatment plants in China.

Introduction

In 1996, the centralized processing rate of municipal wastewater in China was only 11.4%, and will reach to 40% in 2010, which means that municipal wastewater treatment plants (MWWTP) will be increased by over 1000, and most of them will be small and mid-scale MWWTP (Wei, 2000). In 1996, China had more than 100 MWWTP treating 10.22 million m³/day, and produced about 0.5–1.2 million Mg dry sewage sludge and 43.8–83.95 million Mg wet sludge annually (Xue, 1991, Ge et al., 1995, Zhu et al., 1998, Yang et al., 1999). Sewage sludge treatment and disposal has become a pressing issue of concern for local governments throughout China. Land application and landfilling of sewage sludge are the main disposal modes of sewage sludge treatment in China, and land application is and will be the main disposal mode in the near future (Yang et al., 1999). Composting and compost application were optimal for small and mid-scale MWWTP according to a life-cycle analysis in the selection of sludge treatment in 15 small and mid-scale MWWTP, and anaerobic digestion and biogas utilization were suitable for large scale MWWTP (Wei et al., 2000). In China, sewage sludge is directly used for agriculture without any pretreatment because few of MWWTP have sewage sludge stabilization equipment or are equipped for sewage sludge treatment, such as anaerobic digestion (Xue, 1991, Ge et al., 1995, Zhou, 1990, Mao, 1991, Meng et al., 1996). Though there are many municipal solids waste (MSW) composting plants in China, no full-scale sewage sludge composting plant has been built. Land application of sewage sludge is restricted due to heavy metals, pathogens, and persist organic pollutants in the sludge. Declines in available land space, coupled with increasingly stringent national regulations governing the design and operation of new landfills, have caused the cost of siting, building, and operating new landfills to rise sharply. The current legal and economic environment has provided impetus for community leaders to explore alternative means of limiting sewage sludge treatment.

It is essential for MWWTP contemplating establishment of a sewage sludge composting facility to have reliable and current information on the likely cost of establishing and operating such facilities. At present, such information is not readily available for small and mid-scale MWWTP in China.

This article begins to remedy this information gap by analyzing the costs of building and operating sewage sludge composting facilities of different levels of technical sophistication. Such information is necessary for informing local decision-makers as to whether composting makes economic sense, given local fiscal and budgetary constraints. If it does, then comparison of the costs of different composting facilities for a given volume of sewage sludge provides insight into the type of facility that should be constructed.

A variety of composting systems exists, ranging widely in terms of technological complexity. Two basic composting systems are open systems and in-vessel systems (Huag, 1993). In this article we concentrate on windrow, aerated static pile (ASP) and horizontal agitated solids bed (HASB). Windrow and ASP are open composting systems. HASB has the characteristics of forced aeration and pile turning and is the commonly used type of in-vessel composting systems (Wei et al., 1999). Our analysis considers the costs of windrow, ASP and HASB composting facilities of varying technological sophistication operated at three different moisture content of sewage sludge that correspond to localities of different sizes.

The article is organized as follows. The next section provides some background information on the options and tradeoffs involved in the design and day-to-day operation of sewage sludge composting facilities. The third section describes the prototype facilities for which cost estimates are presented. The fourth section outlines the assumption made in computing the cost of constructing and operating prototype facilities and presents the cost analysis of various types of facilities. The final section summarizes our findings.