Fully Automatic pH Correction System

Fully Automatic pH Correction System

Requirements

This fully automatic pH correction system was required as a result of a failure to meet discharge to effluent consent levels. Site visits were arranged to determine a full understanding of the client’s process, gather data to establish an acceptable treatment philosophy and to size an appropriate ergonomic system which performs to the client’s expectations.

It was concluded very quickly that a batch treatment system was more suited to meet expectations and as such the following was derived.

Principle of operation

This fully automatic pH correction system has been designed to treat a stated amount of acidic process effluent produced over a 24hr period.

As process effluent water enters the main bulk storage tank under gravity conditions only, no action will occur until such time as the effluent water level increases to a point at which the tanks “Extra low level” probe is covered by the water.

At this time the level interface relay is energised which breaks the hold on the signal to a corresponding relay. As this relay is “de-energised” the signal is then supplied which will in turn energize the air operated double diaphragm transfer pump (AODD pump).

When this AODD pump is energized, it will quickly start to transfer the first batch of the effluent water over from the main holding tank into the neutralisation tank.

During the commissioning process it was determined that the transfer time should be approximately 20 minutes.

As the neutralisation tank starts to fill, there are certain level probes within this tank which control the remaining functionality from first fill (transfer).

Step one: Neutralisation start process.

As the neutralisation tank starts to fill from the AODD transfer pump, level probes set at a point just above the re-circulation pump outlet valve will get covered by the rising water level. When the probes are covered, the level interface relay will energise, this will in turn energise further relays which will then control the following functions…

  1. The neutralisation tank re-circulation pump will become energized and will start to re circulate the water within the neutralisation tank through the pH controller sample manifold and back into the top of the storage tank.
  2. The individual pH correction dosing pumps will be “enabled.” Both acid & caustic correction pump control logic is contained within this section. Only caustic dosing being connected.
  3. The pH controller (if controlling set points are in the relevant area), will energise either the caustic dosing pump, or the acid dosing pump, dependent on set point.

Step two: Transfer complete.

As the neutralisation tank water level increases, level probes set at the maximum fill height of the tank will be covered. When these probes are reached the level interface relay will energise, this will in turn energise a relay, which will then disable the output to the AODD transfer pump. With the sample recirculation pump still running, the pH correction dosing pump (Acid or Caustic) will dose chemical into the sample/ re circulation line dependent on pH set point.

Step three: Batch “Neutralisation process” complete.

Whilst the transfer process is running, the neutralisation process has already commenced, therefore as the transfer of the untreated effluent water reaches the high level mark, the greater majority of the neutralisation process has already been completed. The dosing pump will continue to operate until such time as the set point is reached, however as the set point gets closer the dosing pump will start to slow down via the external pulse control, as programmed via the pH controller and the proportional controller (Varipulse) mounted directly to the dosing pump. This helps to reduce “Over correction” of the treated effluent water, once the treated effluent within the tank reaches the set point (or above) and has remained at this point for a period of 5 minutes, as set by the “Neutralisation complete timer,” then the latching relay will energise and the following functions will occur simultaneously…

  1. The pH Tank draining indicator will illuminate.
  2. The tank drain (motorised) valve will be energised and will cycle open to drain down the contents of the neutralisation tank.
  3. The tank drain timer will become enabled and will start to time down for a set period.

Step four: Neutralisation tank drained

As the neutralisation tank water level drops whilst the drain valve is open, the various internal level probes will become exposed…

  • As the high level probe is exposed, no other actions occur.
  • As the low level probe is exposed, no other actions occur.
  • As the extra low probe is exposed, the following actions occur simultaneously:
    1. The tank level interface relay is energised, which in turn operates another relay, therefore…
    2. Power to the open contacts of the motorised drain valve is removed.
    3. Further relay is energised which in turn de latches the Neutralisation completed circuits. At this stage the tank drained timer is de energised therefore disabling a potential tank drained fault alarm.
    4. The power is once more applied to the closed contacts of the motorised drain valve to close the tank drain and allow re filling if required.
    5. If the “Extra low” probes in the main storage tank are still covered, then the AODD transfer pump will energise and the whole cycle will start again.

Step five: Completion of cycle

After all of the above has cycled through a number of similar events as described above, eventually the effluent water level will begin to drop down within the main holding tank. As this occurs, the main tank “Extra low level probes” will become exposed. This occurs as the tank level drops below the low level point as the probes are exposed, the tank level interface relay de energises and latches in a delay timer  This delay timer is set up to allow as much capacity within the storage tank to be removed by way of the AODD transfer pump.

Only when this timer has elapsed will corresponding relay become energised which will then disable the AODD transfer pump and thereby stop any further neutralisation taking place until such time as the next campaign starts.

As the next campaign starts and the tank level probes are covered once again, The Level relay will de energise corresponding timer which will allow the process to start once more.

Installation & Commissioning

Once the Client had decided where the receiving tank was best to be positioned for the process to flow unhindered, we then went about the task of sizing the dimensions of the tank so that it would physically fit into the space allotted and based on the access to site.

The correction tank and system was sized within the remaining foot-print of available space with the pumps specified to enable a batch of effluent to be neutralised in a time in keeping with the maximum output of the process divided by a requested safety factor of margin.

The “coffin” shaped tank had to be installed first, followed by the batch correction system.

The dosing pumps and control were adjusted to optimise the addition of caustic as the waste water was pumped over in order to minimise the neutralisation time.

The project took between 3 and 4 days to install and commission.

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