Optimising wastewater treatment

Optimising the wastewater treatment process using analytical sensors with automated cleaning is a cost-effective way to reduce maintenance, improve efficiency and ensure measurement accuracy.

The goal for any wastewater treatment plant is that it operates efficiently and cost-effectively while meeting local, state and federal environmental guidelines. This means that it is important to optimise sensor control and monitoring at each stage in the process. Even more important is measuring the quality, make-up and chemistry of the water throughout the different stages of the process.

Two of the biggest challenges for the application and management of instrumentation in wastewater treatment is the continual need for the calibration of sensitive sensors, such as pH electrodes, and the general maintenance and cleaning of instruments. In this article we’ll look at a typical sewage treatment plant and examine the different types of sensors that can contribute to successful treatment outcomes and reduce maintenance and calibration costs through digital sensor technology and automated self-cleaning.

A wastewater treatment plant is typically completely outfitted with intelligent sensors and measurement instruments to perform the following functions:

• Online analytical measurements (pH, redox, oxygen, turbidity)
• Flow-rate measurements using magnetic field and ultrasound measurement methods
• Ultrasonic and hydrostatic level measurements
• Temperature measurements
• Inlet and outlet automatic sampling
• Pump and blower control (inverters)

When automatic cleaning is available, it is done using compressed air and/or water and triggered by a PLC, a timer or calendars inside the transmitters. The measurement signal is frozen during cleaning and for a dwell time after the process to allow the electrode to acclimatise before signals are transmitted again.

The pre-screening process

The first stage of the process is pre-screening and involves removing solid particles from the raw wastewater. Typically an automated bar screen removes objects such as rubbish, leaves and tree branches. This is followed by a grease and grit removal stage to prepare the influent for primary treatment. Differential level is measured at the bar screen stage while pH, flow and sometimes conductivity are measured at the grease and grit removal stage.

Flow and level measurements are usually measured at the stormwater basin, which takes the overflow of water during periods of heavy rain and surface runoff and releases it when the plant is ready to accept an additional load.

Figure 1: Wastewater plant process overview.

Determining total organic carbon (TOC) during the pre-screening stage gives an indication of water quality and changes to the amount of oxygen that will be required further on in the secondary treatment process. Total organic carbon can be determined either by measuring both the total carbon and the inorganic carbon (IC) present in a sample and subtracting the IC from the total to yield the TOC value, or by removing the IC first and measuring the leftover carbon. The latter process is carried out using pH-controlled acid dosing to remove the IC followed by thermic catalytic combustion with non-dispersive infrared detection of the produced carbon dioxide.

Primary treatment

During primary treatment, the wastewater passes through a primary clarifier, which allows solid particles of material to settle by gravity for removal to sludge treatment. Ultrasonic bed level sensors are used at this stage to measure the level of sludge in the primary settling tank, and to measure the suspended solids at the primary sludge outlet. They are usually applied at an angle of 6° with an operating frequency of 657 kHz for best results.

Suspended solids can be measured with an optical turbidity sensor. Using a sensor that utilises the four-beam alternating light method with scattered light helps to compensate for contamination and ageing of the optics.

The wastewater then moves on to the secondary stage for biological treatment.

Secondary treatment

The secondary treatment stage has more sensor requirements than all other stages in the wastewater process. It involves biological treatment using air and microorganisms to break down organic solids in the activated sludge. The oxygen causes the microorganisms to grow and consume the organic carbon, giving off carbon dioxide, water and excess biomass (sludge), while specific bacteria transform ammonia to nitrate.

Reliable and repeatable measurements - to enable precise control of the blowers - is crucial at the aeration stage, as it consumes the majority of a wastewater treatment plant’s power. Maximising the plant’s efficiency at this stage is a major factor in ensuring that the plant operates economically. Parameters that can be measured prior to, during or after biological treatment are organic content, ammonium, phosphate and nitrate. An ion-selective measuring system, such as the Endress+Hauser ISEmax CAS40D, can be used to measure ammonium, nitrate and potassium levels continuously and simultaneously. This is a low-maintenance, reagent-free measurement. The sensor consists of ion-selective electrodes and a pH reference, and it is installed in an immersion assembly. As the sensor is immersed directly in the aeration basin, the measuring system responds rapidly to changes in concentration, allowing for early control and regulation of the process. Automatic compressed air cleaning prevents the membranes from fouling. On entering the aeration basin, the suspended solids in the influent are measured. Again, an optical turbidity sensor is suitable here as it is also insensitive to aeration systems.

Figure 2: Ammonia and nitrate measurement at the aeration basin.

Accurate, continuous measurement of the dissolved oxygen content is also crucial at this stage to ensure ideal conditions for the microorganisms to do their work. Optical technology is the best choice here, as it offers minimum maintenance and maximum availability with extended maintenance intervals. Using a system with digital communication to the transmitter also means there is no sensitivity to electromagnetic interference.

Various flow measurements are carried out at this stage, including the blower air to the aeration basin, the flow of fat and sand removal, and the activated sludge returning from the secondary clarifier. A vortex flowmeter can provide reliable airflow measurement - this technology is resistant to vibration and temperature shocks, and no maintenance or recalibration is required, since there are no moving parts. For measuring the fat and sand removal, and the return activated sludge (RAS), an electromagnetic flowmeter can be used, which can be installed easily and safely to provide highly accurate, cost-effective flow measurement.

Figure 3: Measuring dissolved oxygen levels at the aeration basin ensures that conditions are optimised for bacteria to thrive.

After aeration, the water and sludge mixture goes into a secondary clarifier to complete the separation of the liquid and solid phase. It is important to keep a fixed level of sludge in the clarifier to ensure an efficient amount of biomass in the system. To measure sludge levels, an ultrasonic bed level sensor can once again be used. This continuously monitors the bed level of the clarification and settling phases.

Part of the waste activated sludge (WAS) is continuously returned to the aeration tank while the remaining sludge is discharged to the thickening process. The amount of WAS is controlled to maintain the correct level of microorganisms in the aeration stage.

Controlled precipitant dosage is also used at the secondary clarifier stage for eliminating contaminants such as phosphorus. Photometric measurement gives an accurate indication of phosphate levels while electromagnetic flowmeters can monitor the flow of precipitant and fat/sand removal.

The separated water then goes for final treatment and discharge.

Effluent line (basic discharge)

In the final stage, controlling the volume and quality of the effluent is increasingly important in order to meet government regulations. The main sensor applications are effluent sampling, pH and turbidity measurement.

Automated effluent sampling can be carried out by a stationary sampler such as Endress+Hauser’s Liquistation CSF48. The appropriate sensors can be selected to measure parameters including nitrates, conductivity, oxygen, pH and turbidity. An integrated data logger records information such as analyser values, temperature of the samples and time of sampling.

Other specific sensors that may be employed continuously at this stage include those for dissolved oxygen, conductivity, ammonia, nitrate, phosphorus, residual organic load and nitrite.

The effluent then travels though tanks filled with gravel and sand for advanced phosphorus elimination or fine filtering with the help of added precipitant. Depending on the load, frequent filter backwash is activated by differential pressure or level measurement. Control strategies based on inline instruments can optimise both the schedule and duration of backwash to save energy and water, and to increase running time.

Final discharge

Before the water is discharged into a river or re-used for irrigation, all microorganisms must be killed. The disinfection process takes place in contact chambers. After chlorination, sulphites are used in the dechlorination stage to ensure no chlorine is present in the final discharge.

A process analyser can provide a final check before discharge. Accurate measurement of total chlorine, phosphate, ammonium and nitrate is very important at the final discharge. This is the last line of defence where a quality measurement can take place. At this point the quality of the water must be within state and federal environmental guidelines.

Bringing it all together

Analytical measuring systems that use analog connections between the sensor and transmitter face a range of technical problems. For example, pH electrodes generate very weak electric currents that can easily be rendered inaccurate or unreliable due to poor connections or cabling between the electrode and sensor cable or external environmental influences like moisture and temperature changes.

Using digital technology overcomes the operational problems of measurements such as pH, conductivity, turbidity, nitrates and dissolved oxygen to ensure interference-free, reliable data transmission between the sensor and the transmitter, and the transmitter can be located further away. Uniform connection and software across all measuring parameters also has the benefit of standardising device operation to ensure plant safety and minimise training costs.

Another benefit of digital sensor technology (such as Endress+Hauser’s Memosens technology) is that it eliminates the need to calibrate at the measuring point. This capability means that sensors can be cleaned, precalibrated and simply swapped over in the field in just a few minutes. This is particularly useful for pH electrodes that need frequent cleaning and recalibration and greatly reduces maintenance. It also means sufficient time can be allowed between cleaning and recalibration, which increases accuracy. Quality and process data can also be stored in the electrode to facilitate predictive maintenance - with associated cost savings.

When choosing sensors for this type of application it is also helpful to choose sensors with self-cleaning functions. This helps to eliminate the time and expense of sending operators out on site to clean electrodes, and significantly reduces maintenance costs. It also allows cleaning to be done more frequently, improving measurement accuracy.

By Jimmy Britz, Business Development Manager - Analytical, Endress+Hauser Australia Pty Ltd.