Getting real with monitoring

A fluctuating population and environmentally sensitive waters posed a challenge for South West Water's suspended solids monitoring. Clive Teobald of Partech Instruments and Helen Richards and Jack Board of South West Water explain

Suspended solids (SS) load is one of the most important aspects of the treatment process as it has an influence from crude sewage intake right through to final discharge. Monitoring final effluent suspended solids is a key parameter for assessing the quality of the effluent in respect of meeting Environment Agency (EA) consent levels. It is also an indicator of the basic performance of the treatment works as any rise in the level of SS will warn of possible process problems upstream of the final effluent point.

By monitoring and recording SS levels at key processing stages, works managers are able to exercise effective control of the plant and ensure that discharge consent levels are adhered to and running costs optimised. South West Water (SWW)'s wastewater treatment works (WwTW) at Exmouth in Devon is a great example of how the investment in an integrated SS monitoring system can contribute to cost effective effluent treatment.

Exmouth WwTW has seen much of its plant refurbished in recent years. Located close to Blue Flag beaches and the environmentally sensitive Exmouth Bay mean that SWW is under constant pressure to ensure that the works does not breach its consent levels.

The WwTW was constructed several decades ago as a primary treatment plant, but biological aerated flooded filter (BAFF) process technology was introduced in the 1990s. The BAFF plant enhanced the performance of the works and presented the opportunity for works optimisation and reduced operating costs.

A key part of the optimisation process was linked to the fact that the works has large variations in the crude sewage flow rates. During periods of dry weather when flow rates are low it is not always necessary to dose chemicals at the inflow and even though the SS content can be high, the UV disinfection process can cope with this loading. It is when the flow is high and the level of SS is correspondingly high that chemical dosing is required. A solution that would ensure economical use of chemicals and contribute to improved plant performance was arrived at in conjunction with Partech Instruments. Improved management of chemical dosing at the front end of the works through the deployment of several Partech Turbi-Tech 2000LS SS monitors was seen as a way of optimising the treatment process (see figure 1). Originally chemical dosing at the crude inlet was based on flow rate, with most of the dosing taking place during the working day.

Eventually more sophisticated techniques were introduced whereby the chemical dosing was linked to peaks in the flow rates, so at times when the flows were higher more chemicals would be dosed. However, when flow rates were affected by high levels of rainfall, highly expensive chemicals would be dosed automatically although they were not needed. By continuous monitoring of flow and SS level using a Turbi-Tech 2000LS and 7200 monitor, it was recognised that the quantity of chemicals could be readily adjusted according to demand and significant cost savings made.

This decision was made following site visits by Partech's team, which was also involved in providing guidance on the other aspects of the treatment process where SS levels were influential. In this way all the critical process stages could be monitored in real-time and savings could be made.

A second Turbi-Tech2000LS was positioned at the BAFF splitter tank to provide a feed-back loop to control the inlet dosing (see figure 2). The position of this sensor records how well the primary tanks are performing the settlement process. If there is too much clean water coming off the primary tanks, operatives need to know the cause. Should the process fall out of its operating range, plant operators are warned via the 7200 Monitor, which has the capability to raise alarms and produce operating trends.

The final part of the regime involves meeting EA discharge consent levels. Before the introduction of the SS monitoring regime, operators had little real-time knowledge on how well the works was meeting its consent levels. Monitoring SS at the outlet was only performed manually with samples being sent to the Countess Wear Laboratory for analysis, so all information was historical. Installing Partech's Turbi-Tech 2000LS at the discharge point enables SWW to be aware at all times as to whether it is complying and if the whole treatment process is functioning correctly.

Since the monitoring regime was introduced, backwashing of the BAFF reactors has been reduced substantially. The use of chemicals for dosing purposes has come under far tighter control. The systems have now been operating for over 12 months. According to reports provided by South West Water, this has had a major impact on the overall performance and operating efficiency of Exmouth WwTW.

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