Dewatered sludge storage emissions control using multistage wet scrubbing

Abstract

ABSTRACT: The City of Los Angeles' Hyperion Treatment Plant is a 16.6 m3/s (380 mgd) municipal wastewater treatment plant with 8.8 m3/s (200 mgd) secondary treatment capacity. Digested sludge, mechanically dewatered by centrifuge, is stored as 20% solids wet cake in large storage bins. Previous studies identified the primary odorous constituents as dimethyl disulfide, dimethyl sulfide, and methyl mercaptan. Earlier pilot tests conducted at the plant indicated that activated carbon and dual‐media adsorption were not cost‐effective technologies for controlling the emission of odors, regulated volatile organic compounds (VOCs), and reactive organic gases (ROGs). An additional pilot test was therefore conducted to test multistage packed towers and various combinations of treatment chemicals. Scrubbing solutions tested included water, effluent, water with surfactants, sodium hypochlorite, sodium hydroxide, sulfuric acid, hydrogen peroxide, proprietary chemicals, and, as a polishing step, activated carbon. Chemical combinations were screened for effectiveness in a preliminary test phase using hand‐held instruments, gas chromatography, and dynamic olfactometry. Based on the results of these trials, more extensive testing was conducted. Analyses included gas chromatography–flame photometric detection (GC–FPD) for reduced sulfur compounds, and gas chromatography–mass spectrometry (GC–MS) for volatile organics. Two methods were used for reactive organics analyses. Total combustion analysis–flame‐ionization detection (TCA–FID), specified by the local regulatory agency was used, as well as nonmethane hydrocarbon approximations using GC. Results were encouraging for controlling wet cake storage odors with wet scrubbing. Removal efficiencies of up to 97% were encountered using a caustic/bleach scrubbing solution. The greatest ROG removal achieved was 89% using acid/bleach and peroxide. VOC removal, however, was poor for most combinations. VOC removal efficiencies ranging from 68 to 81% were recorded when carbon was used as a polishing step. The majority of removal occurred within the carbon stage, however, and not within the packed towers. It is expected that odors and ROGs can be controlled to acceptable levels using this multistage wet scrubbing approach. Additional studies are required to determine whether ROGs are oxidized or simply dissolved in the scrubbing liquid. Removal of VOCs was not encouraging and optimization of VOC removal needs to be investigated further.