FERRIC/FERROUS CHLORIDE INJECTION OPTIMIZATION AS DEMONSTRATED BY REGIONAL CASE STUDIES

In southern California the South Coast Air Quality Management District (AQMD) limits the concentration and mass of hydrogen sulfide (H2S) in gas from all sources (Rule 431.1). Many wastewater treatment plants in the region are successfully applying iron salts to mitigate H2S concentrations in the biogas produced on-site through the anaerobic digestion of biosolids. In the Spring of 2005, the iron salt usage at nine of these facilities was evaluated to assess chemical costs, dosages, and determine the optimum injection locations for application of metal salts to accommodate digester gas H2S regulations.The City of Corona Department of Water and Power (City) is responsible for the treatment of approximately 15 million gallons per day (mgd) of wastewater. The City is considering relocating the ferric chloride injection point from the current site at the primary influent channels to the anaerobic digesters. The projected benefits include reduced chemical costs and improved operations of the primary treatment facilities. The concerns include negative impacts to the digester facility operations, integrity of the digester facilities and potential compliance issues. The City conducted a regional evaluation to investigate these concerns.Findings of this evaluation indicate that relocation of the ferric chloride injection point from the primary influent channel to the digesters: is likely to result in a significant reduction in the chemical dosage required for digester gas sulfide control and the associated chemical costs (5,200 per year per mgd treated); is likely to result in improved plant aesthetics since ferric chloride addition at the primaries causes the formation of a dark (black) sludge with staining potential; is not a common regional practice; may increase the aeration demand of downstream secondary treatment facilities due to anticipated increased loadings; should include an evaluation of the specified chemicals. For example Publicly Owned Treatment Works (POTWs) have observed a dosage gain when injecting ferric chloride into anaerobic environments (digesters). However, the presence of oxygen appears to improve the performance of ferrous chloride, therefore, either form of the iron salt is generally appropriate for application at aerobic locations (primaries). However, certain facilities (Joint Water Pollution Control Plant) have elected to use ferrous chloride when dosing at the digester sites. This option is generally preferred at facilities with upstream dosing locations (primaries and major trunk sewers) to simplify chemical handling operations. Another finding of this evaluation is that regardless of the site selected, the key parameter that controls effective metal salt injection is adequate mixing. A site must be selected with sufficient turbulence, and velocities to ensure an effective chemical distribution and minimize corrosion potential.