Energy optimized control of WWTP in Kaunas
Solution to which problem
The large WWTP in Kaunas serves approx. 385 000 PE. In average 70 000 m³/d wastewater arrive at the WWTP. The biological treatment is designed for simultanoues nitrification/denitrification. An external carbon source is required. The operation of the process requires detailed knowledge. The pilot investment consists of four parts, aiming at increased treatment efficiency. The solutions address a lack of online data necessary for improved control, a potential for improved nitrogen removal by replacement of a methanol dosage point and a mandatory measurement of air flow for improved operation of the blowers integrated with advanced automation of the control of blowers.
The operation of a large WWTP is based on online measurements and automation routines integrated in the process control. Recent developments in the wastewater sector as well as experiences from the operation of Kaunas WWTP led to considerations of small investments which would lead to a future proof solution of operating the WWTP with a high treatment efficiency at low energy demand.
The existing measurement for control of the oxygen setpoints regulated according to ammonia concentration in the wastewater was installed after the sescondary clarifiers. It resulted in process control time lag of ~6 hours.
Methanol was dosed in the beginning of the biological treatment stage – before Bio-P reactors. Therefore, the additional carbon source provided by methanol was not sufficient in a later denitrification stage, as it was partially used by microorganisms other than denitrifying bacteria.
The absence of air flow measurement in the aeration system before the investment did not allow efficient monitoring of aeration membranes. The aeration system of the Kaunas WWTP is of the constant pressure type: whenever aeration diffusor membranes were clogged, the system compensated pressure by reducing the load on blowers.
The following items have been installed/added within the piloting activities at Kaunas WWPT:
- Online nitrogen measurement devices in the aeration tank;
- Airflow meters in the aeration pipes;
- Methanol dosing point replacement in the aeration tanks, directly to the denitrification zone;
- Advanced process automatization for optimising the work of blowers.
The investment started with installation of four E+H Viomax CAS51D nitrate sensors in the aeration tank together with methanol dosing point replacement to the anoxic zones.
Parts of the installation works had to be carried out from a boat in the activated sludge basin.
To further improve nitrogen removal efficiency, an additional ammonia analyzer E+H Liquiline CA80AM was installed. Data from analyzer is used for determining ammonia concentration directly in the active sludge. This is important step in choosing correct oxygen setpoint in the aeration tanks for nitrification.
New mass airflow sensors have been installed.
All these investments were prerequisite for an advanced process automatization, which combines data from all the sensors and regulates oxygen setpoint, methanol dosage, and nitrogen removal efficiency.
The new ammonia analyser provides necessary data to adjust the oxygen setpoint without a delay.
The methanol dosage is optimized based on measured nitrate concentrations from online sensors and dosed effectively to the zone where needed. During almost one year of operation with the new investment Kaunas WWTP improved total nitrogen removal efficiency by 3,2 %, up to total 90,5 % efficiency. New methanol dosing control strategy allowed to use 12 % less methanol than before.
When the new air flow meters have been installed, operators noticed the reduction of diffusers capacity in several aeration sections almost by 50 % in relation to their manufacturers data. Decision was made to introduce concentrated formic acid to the aeration pipes. Formic acid effectively dissolved scalings from the aeration membranes and their capacity returned to normal tolerance limits.
From now on, it is possible to monitor diffuser membrane health, and decide if acid dosing is necessary. Air flow meters data will also be applied for optimizing the work of blowers.
All sensor data (nitrogen sensors, air flow meters) are combined in one system controlling aeration and blowers work, as well as methanol dosing. The advanced control is integrated in the existing process control.