Stable mate for STW bosses

It is widely accepted that managing the solids levels, also known as sludge retention time (SRT) or sludge age, can control the aeration process in a STW. The first SRT controller was launched by US-based Royce Instruments in November 1999 and the system is now being marketed in the UK by Process Measurement & Analysis (PMA), Royce’s partner in Britain.

Royce believes maintaining a constant SRT is important because optimum activated sludge performance can be achieved only by maintaining a constant food to microorganism (F/M) ratio. SRT and the food to microorganism ratio are directly connected by the following relationship: Y (F/M)-k = 1/SRT. Y and k coefficients do not change, unless wastewater characteristics significantly change.

The company also believes automatic SRT control has several advantages over current methods of SRT management;

• automatic SRT control takes into account all changes of solids under aeration and in the waste-stream within an entire 24-hour period rather than taking a snapshot of it once a day;
• the negative effects of lab errors are significantly minimised and SRT calculation errors are avoided,
• total suspended solids (TSS) sampling is reduced by 70-90%,
• significant improvements in sludge thickening can be achieved by maintaining the desired feed mode of thickening facilities.

The SRT system is made up of a number of suspended-solids monitors which transmit data to a fuzzy-logic controller, together with a flow signal. The primary signals are recorded by Royce 7011 analysers, one located in the aeration process supplying mixed liquor suspended solids (MLSS) data, another in the return activated sludge (RAS) line. This is either an insertion sensor or dip sensor depending on plant design. The flow signal is derived from an existing waste activated sludge (WAS) flowmeter, and if required the secondary clarifier blanket level can be fed into the controller.

Individual needs

Each Model 7700 SRT Control System is supplied with control algorithms which have been customised to suit individual plant configurations and tank capacities. The information is gathered prior to supply.

The SRT target for a STW biological oxygen demand (BOD) removal activated sludge process is usually based on achieving good settleability of sludge. Achieving the required degree of nitrification is a criterion for SRT target selection at nitrification plants. The ability of the activated sludge process to sustain shock and toxic loading is often a consideration in selecting an SRT target for industrial plants.

Sometimes plants maintain higher SRT than necessary because of the inability to maintain constant SRT. Automatic SRT control may allow for the reduction in an SRT target, and as a result reduce aeration cost and clarifier solids loading, as well as increasing gas production in the anaerobic digester. It may also stop undesired nitrification during BOD removal in the activated sludge process and improve sludge settleability as well as foam control.

The plant operator can select the target SRT set-point, wasting strategies (i.e mode of operation) and the permissable limits of operation. The controller can handle both continuous and intermittent wasting strategies. Continuous wasting can be set to emphasize either a stable flow or a stable load, depending upon the capacity of the secondary clarifiers. Intermittent wasting can be set-up with either fixed pump time schedules with flow rate control or fixed flow rate operation with variable pump time controls. Pump schedules for the entire week can be entered, with up to four start and stop times allowed each day.

The operator can safeguard the control process by specifying the range of allowable MLSS, flow rate or pump times and load values which should be tolerated. The control system will alert the operator if any of these ranges need to be exceeded in order to maintain SRT. It will also make suggestions as to what ranges might be changed for improved control.

If waste flow or wasting time has to be temporarily changed, in the case of a dangerously high sludge blanket level for instance, the operator can override the controller. Mass of wasted solids in the manual mode will still be included in the controller calculations when automatic waste mode is reinstated.

If the SRT target value needs to be changed, the controller is programmed to alter the value gradually, eliminating the chance of upsetting the system. When an aeration tank is taken off line, it is simple to alter the controller to compensate for the reduced process volume.

The SRT controller is designed to induce stability and reliability, the controller acts as an ‘expert’ system and employs safeguards to detect whether system elements are operating correctly. The operator will be alerted if MLSS, RAS or flow values change in an unusual manner and recommend corrective actions. Nuisance alarms are minimised by smart algorithms which correlate changes in all of the system elements over time to determine if they are reasonable.

The Model 7700 has a 4-20mA output which can be used to directly control a waste pump or valve. A programmable PI control algorithm in the controller regulates waste flow to the target value using the waste flow meter as feedback. Alternatively, a 4-20mA output indicating target flow can be sent to a separate controller, should the user prefer to use external valve or pump speed control hardware. A relay output can be used to start and stop a waste pump, while another relay is wired to an alarm panel to alert the operator of unplanned events.

Royce is the first manufacturer to integrate a group of online analytical instruments from a single source to produce a specialised control system which addresses a specific STW process. The controller’s algorithms are designed to perform continuous automatic solids control within the aeration train, keeping the biomass healthy, the system also provides real-time online control of sludge wasting from the final clarifier.

The primary goal of the system, and main process benefit, is plant stability and biomass health. A healthy biomass, means an efficient treatment process and more cost-effective plant operation. The following operational savings have been achieved;

• reduced clean-up costs after plant upsets,
• reduced chemical usage for Nocardia control,
• reduced chlorine usage due to better overall treatment,
• reduced solids treatment for final wasting.

Because these costs vary between individual STWs, this article does not look at them in great detail. However, the most readily appreciated financial benefit of the system, reduction in the operator time and laboratory analysis costs required to control the sludge ageing process, are common to all STWs. The box adjacent shows how Royce believes these savings are achieved.