Water Journal : Water Journal December 2012
operations water DECEMBER 2012 79 Abstract Aeration is the largest energy-consuming process in wastewater treatment. It is accountable for approximately 40% of total electricity usage in a typical activated sludge plant. Traditional aeration controls maintain aeration tank dissolved oxygen (DO) levels through a fixed main header pressure while adjusting airflow to subheaders with throttling valves. As air setpoint pressure is manually altered higher energy costs are faced, especially during low or dilute inflows. Dynamic air pressure setpoint control (DPC), together with optimised blower scheduling and valve characterisation, was introduced to optimise energy consumption. In 2011, the new control concept was implemented at eight secondary and tertiary wastewater treatment plants in Sydney Water. The implementation included customisation for each plant and training of staff. A power saving of 3% for each kPa reduction in operating aeration air pressure was predicted based on fan affinity equations. The new control strategy at these plants achieved a projected full year saving of $230,000 in electricity cost together with a number of spinoff benefits. A case study of implementing the new control concept at Glenfield WRP is presented in this paper. The case study also introduces a useful indicator for assessing and benchmarking aeration efficiency of wastewater treatment plants. Introduction In 2009, Sydney Water commissioned an energy benchmarking investigation which reported that by improving aeration efficiency by 10%, $350k could be realised in savings. In the same year a program was developed to introduce demand-based or dynamic pressure control to optimise the efficiency of the aeration system. To be suitable for DPC, a treatment plant should have a common aeration manifold supplied by one or more blowers with an air distribution system utilising throttling valves to maintain desired DO levels. In addition to DPC, automatic blower scheduling optimisation was implemented to minimise the number of online blowers at any one time required to meet demand. Valve characterisation was implemented to tighten up control loops. Dynamic Pressure Setpoint Control In essence, DPC is a control strategy that attempts to maintain at least one aeration subheader air throttling valve effectively fully open. As the most open valve starts to throttle, the pressure setpoint is decremented. If the aeration tank DO setpoint cannot be maintained with the most open valve fully open, the main header pressure setpoint is automatically incremented. The pressure balance equation representing a typical aeration treatment process is as follows: PB =P PF +P CV +P D +P H where P B is the pressure produced by the blowers; P PF is the pressure lost across the piping and fittings; P CV is pressure lost across the throttling air control valves; P D is the pressure lost across the diffusers; PH is the aerated water head the air has to push through. DPC minimises P CV and thereby minimises P B . The pressure setpoint is adjusted by means of a modified deadband control strategy. The first step is to identify which valves are the most critical to DO control within an aeration tank system. The valve control loop outputs (in percentage open), which actuate these valves, are then used in determining whether the pressure setpoint is to increment or decrement. Figures 1 and 2 show typical DPC operator setup interfaces. In summary the operators are provided with an interface that allows the following adjustments: 1. Selection or deselection of any DO control loop in the strategy; 2. Display of the current pressure setpoint, which also allows for operator overwriting; A Kapocius A case study of a new control concept at Glenfield WRP AERATION COST SAVINGS USING DYNAMIC PRESSURE SETPOINT CONTROL Figure 2. Typical operator DPC feedback pop-up. Figure 1. DPC setup pop-up.
Water Journal February 2013
Water Journal November 2012-1