Audit of chemical-physical wastewater plant (CP plant)

Audit of wastewater system (industrial, chemical-physical wastewater system)

A chemical-physical wastewater treatment plant for the treatment of industrial wastewater is generally located downstream of production.
Its purpose is to ensure that the regulatory requirements are always met when the treated process water is discharged as wastewater.
The production substances contained there, e.g. acids and alkalis, but also dissolved substances from the production material itself are contained in the wastewater and must be neutralized or removed.
This wastewater treatment process generally worked well at the time of the initial design or commissioning; an audit of the wastewater system clarifies whether this is still the case.
Over the years, however, there are many unavoidable factors that can change the functionality of a chemical-physical wastewater treatment plant.
These include, for example

• Retirement or departure of employees who have looked after the wastewater treatment plant, in some cases for decades.
• The documentation or operating instructions for the system or part of the system such as a selective ion exchanger or other ion exchanger system (such as a demineralized water system or ion exchanger circulation system) can no longer be found.
• The plant is increasingly falling into disrepair.
• Change in the production process (e.g. CrVI process is replaced by CrIII process).
• New production facilities are added.
• The plant has grown and is now subject to the IE Directive 2010/75/EU and new requirements therefore apply; in particular, it is unclear whether the plant still complies with the state of the art.
• Change in the formulation of an individual operating material by the supplier
• New legal requirements to which the wastewater must be adapted
• Unexplained limit value violations or other faults in the wastewater system that may have already been reported to the authorities.
• The indirect discharge permit expires.

In these cases, it can therefore be useful to commission an external audit of the wastewater system in order to obtain clarity about the current situation and, if necessary, to be able to take immediate action.
Depending on the individual case, this can be clarified as part of the wastewater system audit:

1. Approval situation
   1. Is the plant (and any incoming process water flows) sufficiently covered by the existing permit in this form?
   2. Does the permit require an adjustment in the form of a notification or amendment or a new permit?
   3. A “subsequent approval” may be required.
2. Chemical-physical processes
   1. Is the system still able to achieve the required wastewater limit values with regard to its process when operated properly?
   2. Are there new or better or more cost-effective procedures?
   3. Can treatment chemicals be substituted or replaced if necessary?
   4. If necessary, wastewater treatment tests are carried out in a laboratory phase.
3. State of the art or generally recognized rules of technology
   1. Does the structure of the system still correspond to the state of the art or, if sufficient, to the generally recognized rules of technology?
   2. Are the mechanisms for procuring and stocking spare parts adequate?
   3. If necessary, the system is adjusted.
4. Documentation
   1. Does the documentation still meet current requirements?
   2. New documentation may be required.
5. Training level of employees
   1. Are the employees adequately trained in handling the system, hazardous substances, chemical processes and environmental law?
   2. If necessary, employees will receive additional training.

  Example of a wastewater treatment plant audit (extract): The wastewater system (continuous flow system) is now 40 years old and consists of several steel components and various plastic tanks. As there is UV radiation in the room, the PP plastic tanks + PVC piping should be replaced due to material fatigue caused by embrittlement and also the control system due to age/no longer state of the art.

The average wastewater volume is 35 m³/day, but on peak days up to 100 m³/day (= 4.1 m³/h max.
with 4-shift operation.

The design parameters for a continuous flow system of this type are as follows:

• Reaction times in the treatment tanks: 30 minutes for precipitation and flocculation with sodium hydroxide solution
• Stirrer selection in the treatment tank: A gear mixer of approx. 80 W/m³ or the tank contents should be circulated twice per minute.
• Reaction time of the flocculation tank: 30 minutes.
• Reaction time in the settling tank (sludge-clear water): 45-60 minutes.
• Filter pressing: Continuous automatic emptying or emptying once per shift.
• Filter press pump: At least 12 times larger than the filter volume of the press.
• Multi-layer filter: closed design with a filter speed of less than 10 m/h.

The “2 m³ treatment tank” only has a volume of 1.6 m³ due to installations and agitator and is therefore too small and should be enlarged to an actual 2 m³, preferably 4 m³, in a new design.
Any monitoring values that are exceeded are mainly due to the insufficient contact time.

The two storage tanks for the waste water are each 15 m³ in size.
Experience has shown that there is not particularly good mixing in the tanks, i.e. there are always strongly acidic and strongly alkaline components, so an agitator would be an advantage.
This would homogenize the pH value in advance and reduce the load on the actual treatment tank.
The disadvantage, however, would be that precipitation may already occur.
However, this could be utilized with an inclined bottom for complete emptying or at least for hydroxide discharge.
However, a diaphragm pump would then be required for transportation, as the existing centrifugal pumps would quickly become clogged.

Typically, the authorities require the separation of acidic and alkaline wastewater for mutual neutralization as a chemical-saving measure.
However, as the alkaline wastewater is reported to be very low and only occurs occasionally, this can be dispensed with for this reason.

The “preparation stations” for other chemicals (_FeCl3 (liquid delivery) + Zetag 8180 flocculant (delivery form: solid)) are showing considerable signs of wear and should be replaced.
The AwSV-compliant design must be observed, as these are AwSV storage tanks: for Zetag 8180 a PP preparation tank with sloping bottom as before, which is completely transferred to a new PE storage tank with trough and DiBt approval.
The FeCl3 tank would also have to be designed as a PE storage tank with sump and DiBt approval, refueling as before from the WHG sump with the chemical pump.
The sump on the _FeCl3 tank is also too small, § 18 Para.
1, 3 No. 1 AwSV.

Note: It used to be possible to use simple PP containers without approval if they only had a single approval.
In new plants, however, these options are no longer regularly approved by the AwSV experts; instead, AwSV-compliant storage containers are required as described above.
Exceptions are genuine dosing tanks, i.e. if the entire contents of the tank are emptied in one shift / 24 hours.
It is also possible that these had an earlier VaWS SV individual approval, but nothing is noted on the containers and no other documentation is available.

The lamella separator is made of steel and can and should be retained.
The chamber filter press is still an open chamber filter press and could either be designed as a closed one (no splashing, no more odor formation, acid rinsing of the plates inside the press possible) or as a closed one with automatic emptying.
The use of an FU pump would make sense here and would fulfill the requirements of § 3 Para.
2a AbwV.

The small 100-liter chemical dosing tanks on the treatment tank should be omitted anyway, but they would be permissible.

The multi-layer filter is a PE-GRP container, which could also be designed as a steel filter with a PA inner coating.
The background to this is that a layer of hydroanthracite would also have to be filled into the filter, which would have to be refilled again and again because it can be rinsed out very easily.
However, the container has to be opened frequently to do this.
However, this is difficult to do with the PE-GfK containers because the thread wears out quickly.
For the feed pump of the multi-layer filter, a frequency converter pump would also make sense in order to fulfill the requirements of § 3 Para.
2a AbwV.

A selective exchanger is not currently available, but can be provided by the authorities if necessary.
However, there are chemical parameters that contradict this: the wastewater contains manganese, which oxidizes to manganese IV (manganese dioxide) in an already slightly neutral-alkaline (from pH 6.5).
This forms chlorine gas with HCl during the regeneration of the selective exchanger.
The selective exchanger would therefore be inadmissible for health and safety reasons.

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