The major part of the wastewater for neutralization comes from the regeneration of the brewing water treatment and from the CIP plant. The CIP purification comprises mainly the brew containers, the fermentation and yeast tanks, pipings and a filtration plant. As purification reagents, herefore, by turns, the 2 % sodium hydroxide solution with additives and sour detergents are used. The regeneration wastewater from the brewing water treatment all contain, in contrast, hydrochloric acid. Together with the pre-rinsing water and the production outlet water these effluents reach the neutralization plant. The specific wastewater flow rate can reach up to 10 m³/h.
Neutralization plant
The task of the neutralization plant is to stabilize the pH value of the wastewater pending temporarily strongly between 2 and 12, in the pH value range admissible for the direct line (pH 6.5 to 9.5). For first making use of a two-way advance neutralization of the sour and alkaline effluents and at the same time reaching a balancing of the wastewater flow, the various flows are directed in an external 15m³ stainless steel tank. After the advance neutralization the pH value can still vary between pH 4 and pH 12. From the tank, the outlet is effected in regular operation with a spillway located at approx. 7 m³ fill level into the actual neutralization tank. The tank is made of polypropylene and has a total volume of 2m³. If the fill level does not reach the spillway, a further outlet by means of an eccentric worm pump (8m³/h). The tank is placed in a container together with all components necessary for the neutralization control. A high-speed stirrer provides the homogeneous mixing of the dosed neutralization reagents with the wastewater. A decrease of the pH value can be reached with concentrated nitric acid (53%), a 15% sodium hydroxide solution will result in an increase of the pH value.
The control is effected with a pH value measurement in the homogenous mixing
zone of the neutralization tank. The measuring value is transmitted to a controller with PID control function that is installed at site. The threshold values of the pH value control are situated between pH 6.8 and pH 9.2 with a neutral zone between the threshold values. One of the two metering pumps is controlled with dependence from the measuring value to dose neutralization reagents. On the drain pipe of the neutralization tank, a control measurement is effected with a second pH value and temperature measuring point monitoring the proper course of the neutralization. All measuring signals and operation states are transmitted to the central control room of the brewery.
In case of extreme pH values at a high flow rate of the wastewater, the integrated volume flow control constitutes an additional safety step for the proper neutralization. In critical cases, the limitation of the flow via the spillway will be achieved by means of MID and controlling the check valve via a controller integrated into the PLC.
In order to enable highest reliability levels and maintenance-free operation of the metering-control-circuit, proven and ideally integrated components of one single producer are used.
Readings recorder
The readings recorder used for pH value adjustment and control is a pH electrode type PHEX 112 SE. It can be used in a measuring range between pH 1 and pH 12 up to a temperature of 100°C and a maximum pressure of 6 bar. The pH electrode has a large surface ring diaphragm and a gel electrolyte. Thus, this pH electrode has a very long operating life even when used for polluted wastewater. The optionally available, plug-in measuring transducer PHV1 transforms the primary voltage signal into the 4-20mA standard signal, that can be transmitted by means of two conductors and easily be connected to clamps. The special advantage of this 2 wire technology is obviously the failure-free transfer of the signal, even over long distances.
Controller
The control device type D1Ca applied in the control circuit enables the following functions:
- processing, display and analysis of the 4-20 mA standard signal
- PID control device
- control of two metering pumps
- programming of one alarm and two limit value relays
- 2 free programmable 0/4-20mA standard signal outputs
- control input “pause” for engaging/disconnecting the entire control
- connection of a further D1Ca device (see final pH value control)
Metering pumps
The neutralization reagents (53% nitric acid and 15% sodium hydroxide solution) require high degrees of chemical resistance of the materials in touch with the medium. For exact and maintenance-free continuous dosing, diaphragm metering pumps type Beta 4 and Beta 5 are used with special material for liquid end and sealings. Both pumps are equipped with an automatic ventilation function for outgasing dosing media, an alarm relay (gradient) and variable suction lances with two-stage level switch (PVDF).
Final pH value control
The measuring station for the final pH value control consists of the above-mentioned pH electrode PHEX 112SE and a Pt100 temperature sensor. Both readings recorders are connected to the measuring device D1Ca that contrary to the above-mentioned D1Ca controller has no outputs for the pump control. Instead this device allows compensating the temperature influence on the pH value on the basis of the measured temperature. By connecting the two D1Ca devices, the temperature influence on the pH value measurement of the control circuit can be compensated.
Résumé
The above-presented regulation of a neutralization task exemplarily shows the design of a control circuit for dosing reagents with ideally integrated components that are adjusted to the respective application. The right composition and execution of the applied components comes up to the know-how of the plant manufacturer and is as important for reliable and maintenance-free operation as the high quality of the components itself is. The customer benefit is always the first target of all efforts, as in the present case the direct discharge of the neutralized brewery wastewater into the municipal wastewater system. |