Our approach to shut down of the physical-chemical treatment process

Stopping a physical-chemical treatment generally does not present many problems nor differ substantially from the activities during a (regular) maintenance shut down. For example for equipment such as pumps, pipes and heat exchangers the risk of clogging by hardening of the product must be avoided. Depending on the medium, equipment must be rinsed and/or drained. Since it is difficult to predict when production processes will be restarted, it may be necessary to preserve process equipment for a longer period and protect them against biological growth, such as the preservation of membranes and other filters and the prevention of legionella.

Our approach to shut down of biological treatment units

The way in which both aerobic and anaerobic treatment units are turned down before a major shut down has a major impact on the speed of the start up process when production is restarted. There is an incorrect assumption that treatment capacity can be maintained, when the treatment plant continues to run at low capacity and a concentrated feed stream is added. However, this does not apply if the purification process has to be stopped for more than a week.

The reason for this is that the bacteria in a biological treatment process are constantly adapting to new process conditions. If less food enters, the bacteria remove the excess enzymes with which the conversion processes are carried out. The biomass becomes “lazy” and the conversion process decreases. In fact, if no nutrition comes in at all and in the case of aerobic treatment the aeration remains, the bacteria will digest themselves and the potential conversion capacity will decrease greatly. We call this process aerobic sludge digestion.

Applying an external feed source apparently keeps the capacity in order. However effluent concentrations actually increase should specialized bacteria disappear by the time capacity increases to normal conditions.

We strongly advise to:

  1. Maintain nutrition levels
  2. Stop aerating the aerobic processes
  3. Maintain a steady influent source

The best way to prepare the biomass in the treatment process for a long standstill period that is often followed by a quick start up process, is to keep the feed before the standstill at normal levels for as long as possible. When there is no more wastewater or the influent buffer has reached the desired minimum level, the feed can be turned off completely.

Shut down for the anaerobic treatment process

Turn off the recirculation, pH regulation and nutrient dosing only when the gas production is less than 10% of the original gas production. All controls can then be stopped.

Shut down for the aerobic treatment process

It is still necessary to aerate for about one day. All recirculation and dosing can then be stopped. During the first two weeks of standstill, the aeration should be switched on briefly two to four times a day with a total aeration time of about one hour per day. In the unlikely event that the shut down lasts longer, a treatment-specific approach may be necessary.

Environmental or discharge permit

In the context of the environmental or discharge permit, we recommend you report to the competent authority that no effluent or effluent of a different quality will be discharged temporarily. This means, for example, that a temporary reduced load (during the standstill period) or increased load (upon restarting) also has to be taken into account in the (post) treatment process. In the case of discharging directly to a surface water body, coordination with those competent authorities will always have to take place.


Hopefully the factory shut downs will only last a short time. As soon as it becomes clear that wastewater is re-entering the plant and the buffers have been filled again to a reasonable level (e.g. 50%), the treatment can be restarted gently.

Once you have followed our stop and shut down strategy, you’re welcome to follow our start up description below as a reference guide.

Remember! Each treatment process contains equipment and controls with its own specific requirements / steps.

You no doubt have a (re)start up plan. Review it critically. We are here to help you with that.

Before restarting the wastewater treatment process, make sure that the installation equipment is ready to be put back in operation (cleaned, refilled, serviced).

Temperature correction

For most treatments processes, the process temperature would have decreased during the standstill period. Each degree of temperature decrease means an activity loss of 8% for the biomass. It is strongly recommended, if possible, to heat up the treatment plant just before or during start up. If this is not possible a temperature correction will have to be carried out.

The start up of anaerobic treatment plants

Anaerobic treatment plants can re-start at 80% of the final capacity (after temperature correction). It is wise to slowly increase the influent flow during the day so that the 80% is only reached after 24 hours. Results of the fatty acid analyses in the effluent must show whether this start up is going well and whether the load can be increased further.

The start up of aerobic treatment processes

In Aerobic treatment processes the aeration and recirculation can start one day before the start of the feeding. The speed of start up is then strongly dependent on the presence of biodegradable toxic components. If these cover a large part of the total COD, we recommend you start with 25% of the final capacity (after temperature correction). With easily biodegradable COD and nitrification, you can start with 75% of the final capacity (after temperature correction) and even scale up to 100% within a few days.

Naturally, there are treatment processes which will have a very different influent composition caused by the turning down and ramping up of capacity. In such cases, a specific approach may then be necessary including temporary storage and / or external wastewater treatment. We are here to advise and support you on this.

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