Chemical characteristics of an anthropogenic marine sediment and its biogas production potential

Abstract

A strip of 30 hectares of coastal water along the Hong Kong's Victoria Harbour had been heavily polluted by industrial as well as sewage discharges for over 30 years by the 4 million residents on both sides of the Harbour. A series of chemical analyses were conducted to characterize the degrees of contamination with an emphasis on those parameters that may relate to odour generation, followed by monitoring of biogas production. Core samples up to 6 meters depth were collected at six sites. Each core sample was then analyzed at seven depth levels for various parameters, including heavy metals used for electroplating (i.e. chromium, cadmium, copper and nickel), pH, total solids, volatile solids, total organic carbon (TOC), chemical oxygen demand (COD), acidified volatile sulfide (AVS) and methanogenic activity. Analytical results showed that sediments at inner sites contained more toxic substances, such as chromium and COD, and produced more methane than those at outer sites. Sediment at top layers contained more organic matter but less toxic pollutants, and produced more methane and hydrogen sulfide. Although the sediment is heavily contaminated with heavy metals, most of these metals are likely in the form of stable sulfide precipitates that are not bio-available. Hydrogen sulfide is the main source of odor generated by the sediment, resulting from sulfate reduction and degradation of organic sulfides. The six sediment sites produced a daily average of 6 L/((m2-day) of biogas, including 2 L/((m2-day) of methane and 0.2 L/((m2-day) of hydrogen sulfide. The average sulfur content in hydrogen sulfide produced on average equaled to 1.1% of AVS content in the sediment. The rate of hydrogen sulfide production increased with the biodegradable organic content, but was independent of AVS in the sediment and the depth level.