A Mass Balanced Approach to Process Control of a High Density Sludge Process

The high density sludge (HDS) process was developed for neutralization and precipitation of heavy metals found in iron-bearing acid mine drainage, as well as in diluted waste pickle liquor from steel finishing operations. The HDS process removes dissolved heavy metals in wastewater through chemical precipitation. This process uses an aqueous alkaline neutralizing agent that is introduced with a carrier in a completely mixed reactor, followed by a second completely mixed reactor where the mixture is combined with an acidic heavy metal containing wastewater. The resultant mixture is separated from water in a gravity sedimentation device (e.g. clarifier) prior to the discharge of the treated wastewater. The HDS process reduces the sludge disposal volumes, and results in improved heavy metals removal as compared to other conventional precipitation processes. Compared to conventional precipitation processes the HDS process is subject to high effluent Total Suspended Solids (TSS) because of the high solids loading rates of the clarifier. Given this, there are some unique operational challenges associated with HDS process control.CH2M HILL and Operations Management International, Inc. (OMI) commissioned and operated an HDS wastewater treatment plant (WWTP) at a steel finishing mill. The steel finishing mill contained galvanizing, cold rolling, pickling, and annealing operations. Pickling of both carbon and stainless steel rolled strip was performed resulting in the discharges of wastewater containing hydrochloric, hydrofluoric and nitric acids in addition to chrome, nickel, and iron heavy metals. Initially, the conventional operations process control strategy was employed for the HDS WWTP.Conventional HDS process control methods have consisted of performing a combination of laboratory analyses (e.g. TSS, solid formation test, total metal hydroxides) coupled with unit process operation information (e.g. flows, pH). This information is assimilated and input into an operations control spreadsheet where qualitative targets are applied. The output is then used to change the HDS process variables that stabilize the process and/or result in a nearly constant sludge density. These variables are: neutralization pH for principle heavy metals, coagulant/flocculant feed, recycle rate of carrier solids and wasting rate of clarifier solids. The time consuming nature of these tests and input/output process limit the ability of the operator to make real time changes to the system variables, thereby limiting performance of the HDS WWTP.Given the highly variable wastewater constituents and flows from the finishing mill, and the inherent limitations to the conventional HDS process control methods, an improved process control method was developed to streamline performance of the HDS WWTP. A quantitative mass balanced approach was developed to control the HDS process. The method utilizes faster analytical methods and real time process information to enable the operator to change the process in a timely manner to stabilize the process and improve the performance of the HDS WWTP. The improved HDS process control methods and the resulting performance is described this paper.