Waste water management Description Description

Many chemical sites operate a central WWTP as final treatment step. In most cases, such final treatment is based on biological treatment, but sometimes only physico-chemical treatment is used.

Operating procedures

The operation of a central WWTP is governed by operating procedures linked to the procedures/EMS (see Section 3.1.2) of the individual plants/units from which the waste waters originate. These procedures describe in particular the necessary interactions between the operators of the individual installations producing the waste waters and the central WWTP operators. When operators of the individual installations and operators of the central WWTP pertain to different companies, conventions are set up (see Section 3.1.3). Adequate communication between the production plants and the central WWTP must be ensured (e.g.

radio, mobile phone, PC network) for normal operations and incidental or emergency situations.

All plants/units or even areas of the chemical site have local emergency procedures for ensuring that leaks of harmful substances are appropriately dealt with; this generally involves containing the release in sumps or intermediate bulk containers (IBC) and notifying the environmental section so that they can assess these waste water streams for disposal on or off site.

Managing fluctuations

The operators of a central WWTP have to manage the fluctuations of the influent to the central WWTP to cope with the seasonal variations (summer/winter), start-up and shutdown

productions (e.g. fine chemicals) and other conditions that can affect the composition of the influent (see also Section 3.3.2.1).

When the production of chemicals is organised in campaigns (e.g. in the production of fine organic or inorganic chemicals), procedures are in place to evaluate the impact of the production changes on the waste water streams sent to the sewer to optimise the use of the central WWTP and to minimise the impact on the environment. Process water effluents are analysed routinely from campaigns to check for similarity. Changes to the monitoring strategy/procedures/equipment may be needed if the production (e.g. process, equipment, product) is modified (see also Section 3.1.5.3.2).

As a result of all the possible operational issues that might affect the quality and/or the quantity of the influent waste water to the central WWTP, its operators will observe a wide range of emissions for many different parameters. In order to optimise the performance of the WWTP with the changing influent conditions, information is needed on the limitations of the WWTP design (e.g. hydraulic capacity, type of aeration, aerobic and/or anaerobic treatment steps), the interference to the biological community/biocoenosis and the local operating conditions.

Knowledge of the individual waste water streams to be treated

Effective waste water management on a large chemical site requires knowledge of the relevant parameters in the local situation, e.g. mass flows, concentrations of specific single substances or sum parameters, temperature, pH, as well as flexibility in handling different circumstances and relations between different waste water streams coming from the production plants/units.

Establishing and maintaining a stream inventory/register (see Section 3.1.5.2.3) allows for identifying the parameters that can have an influence on the performance of the central WWTP and to take all the necessary actions in order to ensure its proper functioning.

For each waste water effluent sent to the central WWTP, the operator of the central WWTP and the operator of the plant/unit from which the effluent originates collaborate to set up waste water specifications which define, in particular, the range of each relevant parameter for the effluent (e.g. flow rate, pH, temperature, biodegradability and bioeliminability, solid content, toxicity to biomass and to the aquatic environment, heavy metals, colour) as well as the nature and frequency of the monitoring regime for the effluent.

As long as the waste water quality is within specification, it can generally be discharged to the sewer system without further communication between the operators of the plant/unit and the operators of the central WWTP. When the water effluent is off-specification and this is identified, the operators of the plant/unit from which the effluent originates contact the operators of the central WWTP in order to decide how to handle this new situation. Waste water may be sent to the sewer without any pretreatment, if the treatment capacity of the central WWTP allows for it (in terms of the nature and amount of the waste water capable of being treated for the specific situation). In any other case, waste water is temporarily stored in buffer tanks or emergency storage basins. The emergency basins can serve as buffers to regulate the input to the sewer in order to remain within the capacity of the WWTP. In the emergency storage basins, pretreatment is feasible.

Testing waste waters before decision for treatment

Before new waste water conditions are accepted for it to be discharged to the sewer network of the site, they need to be examined and/or tested. Tests are carried out under the supervision of analytical experts from the WWTP and in representative conditions with the original waste water from the actual central WWTP. The effluent is tested including using biotests, if needed (e.g. a Zahn-Wellens test can be used). If the new waste water cannot be accepted, pretreatment may be needed or disposal will be required.

Monitoring of the sewer network of the chemical site

monitoring frequencies (e.g. continuous, grab sampling) for an efficient waste water management.

Some examples of important parameters to be monitored include:

 the mass flow of waste water;

 the temperature of waste water;

 carbon, nitrogen and phosphorus and the relationship between these parameters in the aeration step of the central biological WWTP;

 pH;

 critical (single) substances from single plants/units;

 sludge activity (temperature, feed concentration, retention time) in biological WWTPs.

Achieved environmental benefits

Optimised performance of the central WWTP, avoidance of poor environmental performance as a result of fluctuations in the quality and/or quantity of the influent to the central WWTP.

Cross-media effects None.

Operational data

The treatment process of a central biological WWTP consists of mixed microbial communities which are able to degrade the pollutants under certain operating conditions. In order not to deteriorate the performance of the biological treatment process, the operators of the WWTP have to make sure that the operating requirements are met at all times. In case of disturbances to the performance of the biological treatment, the operators have to take necessary actions to manage the specific situation efficiently and in a timely manner. As a minimum requirement, the C/N/P ratio in the influent has to be ensured under all operating conditions. Therefore, the operators of the WWTP have to have information on the influent waste waters and on the operation of pretreatment units inside the chemical plants. In case of the lack of a carbon source for denitrification and/or the nutrients (N, P), the operators should supply a feed (e.g. side products, concentrated waste water stream) to compensate for the deficient amount. With respect to maintaining the C ratio in the central WWTP, different wastes containing a carbon source can be used, and therefore, the degradability of the waste substances in the biological treatment determines whether or not it can be used as feed.

In order to ensure a good performance for the biological treatment of the biological WWTP, the C/N/P ratio, sludge age, structure of sludge flocs, adaptation time to variation of influents, toxic and/or inhibitory influence should be kept under control.

If a pretreatment unit (e.g. distillation unit) for reducing ammonia nitrogen (NH4-N) is installed and operated in a chemical plant/unit in a situation where the central biological WWTP may need TKN (C/N/P ratio), the operator of the central WWTP may ask to not operate the pretreatment unit and send the ammonia-rich waste water to the sewer system. In that case, the central WWTP does not need to use additional nutrients, e.g. urea or HNO3, the operator of the chemical plant/unit saves energy for the pretreatment of waste water, and no waste is generated.

A high concentration of nitrogen in the single waste water stream can be allowed if the mass flow is balanced within the whole waste water influent and homogenised in the buffer before aeration (WWTP #21) [ 222, CWW TWG 2013 ].

Waste water from the blowdown of closed cooling circuits which contain phosphorus may be discharged to the central biological WWTP for the same reason (C/N/P ratio). The use of an additional feed, e.g. of HPO , may be avoided and the direct discharge of phosphorus to the

Dealing with non-routine waste water arisings

Non-routine waste water arisings (i.e. side products, failed production runs, etc.) are assessed by the environmental section for release to the sewer, based on information that includes:

 health and safety data (to protect plant personnel, especially those working in the central WWTP and on the sewer network);

 aquatic toxicity data;

 biodegradability/bioelimination data of each waste water stream;

 odour threshold;

 the removal efficiency of the central WWTP.

Waste streams are only released to the sewer when it can be shown that the central waste water treatment processes of the site can cope with the discharge. If there is any doubt, waste water streams are sent off site for disposal or to temporary storage for verification before a decision on treatment is made.

Applicability

Applicable to all chemical sites discharging to a central WWTP.

Economics

No information provided.

Driving force for implementation Efficient waste water management.

Example plants

WWTP #21 [ 222, CWW TWG 2013 ].

Reference literature

[ 222, CWW TWG 2013 ] [ 227, CWW TWG 2009 ]

3.1.5 Management tools