Optimised chemical dosing results in savings for wastewater plant

With a continuing task of maintaining the good condition of the Glatt — a river that was once significantly contaminated — the Bachwis wastewater treatment plant in Fällanden, Switzerland is constantly striving to optimise its operations.

The plant processes the wastewater of the four connected communities of Volketswil, Schwerzenbach, Fällanden and Maur. Designed for a population equivalent of 45,000, it treats a sewage volume of more than 5,500,000 m³ per year. A mechanical cleaning system transports the wastewater into the two-line primary sedimentation system. The water is biologically cleaned in the anoxic tanks and the three downstream aeration tanks with a post-aeration tank using alternating/intermittent aeration. The treated water is pumped from final sedimentation into a filtration facility before it is finally introduced into the river Glatt.

 

Figure 1: Bachwis wastewater treatment plant flowchart — click here to view a larger image.

Problem

Previously, dosing of the precipitant was adjusted manually based on the laboratory measurement value of the daily composite sample and so was unable to respond to possible peaks. Although being compliant with the effluent limits, the values observed fluctuate between 0.2 and 0.8 mg/L.

The precipitant was added at two points — to the return activated sludge and upstream of the filter. Dosing of the precipitant into the return activated sludge was adjusted using the laboratory measurements of the daily composite samples. Where the last laboratory samples showed an increasing or decreasing trend, the dosing quantity was increased or decreased accordingly. The dosing quantity in the filter was consistently maintained at 2 L/h.

Following an assessment of the plant’s potential, it was revealed that further optimisation of the use and consumption of FeCl₃ was possible in the area of precipitant dosing.

A more detailed analysis of the operating data was then carried out in collaboration with Hach consultants. The plant was visited in order to gain an accurate insight into the current situation.

Solution

An initial proposal was to shift the variable precipitant dosing from the return activated sludge to the final aeration of the alternating/intermittent tanks. A two-channel phosphate analyser with two sample preparation modules and the RTC-P control module for two dosing points was also recommended.

As a result of the initial proposal, initial optimisation and savings could be achieved even before the installation of the RTC-P control module. A six-month test phase was agreed and the equipment required for the optimisation process was installed.

Figure 2: RTC-P set-up diagram.

The measured values and control values of the analyser and the RTC-P are first transmitted to the central control system, where the dosing can be controlled. This has the advantage that, in addition to the existing fallback level of the RTC-P, control of dosing can also be activated or deactivated at the control system (eg, when tanks are being cleaned). As the measurement is taken after dosing, the RTC-P was set to ‘feedback control’ with an initial set point of 0.5 mg/L.

Following the initial set-up, parameterisation of the system was continuously monitored via the GSM module of the SC1000 controller and adapted in each case in consultation with personnel at the wastewater treatment plant. It quickly became evident that the PO4-P measurement with the Phosphax sc analyser at the plant provided additional useful data which had not been available previously. The decision was therefore made to keep the measurement long before the test phase ended. After the parameterisation of the RTC-P module underwent additional adjustments, the module also demonstrated its usefulness and was acquired by the plant.

The module has now been running successfully for three years and can be operated by the plant personnel themselves if the parameterisation needs to be adjusted. The final steps carried out by Hach included installing the predictive diagnostic system Prognosys, as well as the new software for the RTC-P module. The new version of the software fulfils the requirements of the operations manager, who wanted the system to respond even more quickly to phosphate peaks — a specification that has been achieved with the update.

Figure 3: Time curves from Biology 1 — click here to view a larger image.

According to the operations manager at the plant, Martin Moos, the latest version of the software is helping the system to work even better than before. “The module is very intuitive to use and any necessary changes to the parameters can be carried out by my employees quite easily. The Phosphax sc analyser complies very well with the laboratory values and does not require any time-consuming maintenance,” he said.

Results

Since the P module controlled dosing at the final aeration stage so effectively, dosing on the filter was initially reduced before finally being stopped completely. Compared with 2011, the 17% saving achieved in 2013 even exceeded the estimate made by Hach (10–15%). The measurement values of the daily composite samples in the procedure vary within a much narrower range of around 0.4–0.75 mg/L compared to the previous range, which also means that the limits are always respected.

In the first year following the installation, the minimum dosing quantity was between 6 and 7 L/h. As personnel have built up their knowledge and gained experience of the module, this figure has now been reduced to 4.5 L/h, resulting in additional savings.

The latest version of the Hach RTC-P System is now available in Australia and New Zealand.

Top image credit: ©iStockphoto.com/Ashok Rodrigues