Real Time Control of Caustic Dosing Through Online Ammonia Sensors

The wastewater entering the Mallard Creek Water Reclamation Facility contains insufficient alkalinity to meet the 1.0mg/L summer and 2.0mg/L NH3-N ammonia limits. To supply enough alkalinity, the facility began dosing sodium hydroxide into the primary effluent. In fiscal year 2007, the facility exceeded the budget for caustic as the price rose from 0.61 to 1.11 per gallon. Staff theorized that setting the caustic dose via a daily grab sample was leading to a significant overfeed, resulting in wasted caustic and wasted dollars. Determined to find a better solution and stay within the budget, the staff sought to determine if an online ammonium sensor would be reliable enough to control the caustic dose. After six weeks of testing, the sensor was proven reliable measuring in the primary effluent, and a real-time caustic dose was implemented based upon the ammonia concentration. Compared to the fixed dose mode of operation, the real time method reduced the daily dosage by 300 gallons of sodium hydroxide, equating to an annual savings of 68,620; or a return on investment of approximately five months.