Buffer storage for waste water incurred during other than normal operating conditions operating conditions

Description

Other than normal operating conditions such as operational failures, equipment leakage, unintentional contamination of cooling water, or other disturbances in production or storage units can lead either to an increased discharge of pollutants into the receiving water via the WWTP, or to its malfunction. The risk of such events can create the need for centralised or decentralised receiving (or buffer) facilities. For the operation of a barrier or buffer system, timely detection of the failure event is crucial. This detection can be ensured by both analytical and organisational means [ 251, Ullmann's 2000 ].

Achieved environmental benefits

The technique allows for lower emissions to the receiving water.

Cross-media effects

There are no cross-media effects associated with this technique.

Operational data

There are several buffer devices available. Their capacity has to be sufficient to store all waste water, probably including rainwater, incurred during a production failure. They can be combined with flow and load balancing tanks (see Section 3.3.2.1).

The appropriate buffer storage capacity is usually based on a risk assessment (taking into account e.g. the nature of the pollutant, the effects on further treatment, and the receiving environment.

One device (see Figure 3.6), the self-contained buffer, contains two buffer tanks that receive the waste water stream alternately. While one tank is filled, the content of the other is checked and then released to the waste water discharge or downstream of the WWTP or disposed of as waste, depending on the results of the check. The receiving capacity of each containment needs to be sufficient to hold the entire amount of waste water arising during the period of analysis and the emptying of one tank. For complex and large chemical sites with large amounts of waste water, this is often an option for selected waste water streams, because otherwise the requirement for tank volume is immense. The larger the tank, the longer the emptying time and vice versa, which might result in a vicious circle.

Another device is the connected buffer, either discontinuously (see Figure 3.7) or continuously filled (see Figure 3.8). The discontinuously filled buffer is disconnected when not in use, i.e.

when there is no failure reported by the control and alarm system. During normal operation, the waste water circumvents the buffer system, and only when the control system detects an unusual event is the buffer tank filled. The required tank capacity is the amount of waste water arising during the time of malfunction. This device is used with single production installations, a collection of selected waste water streams and with the total amount of waste water. The required volume is normally much less than with the self-contained buffer mentioned above.

Sewer system

Alternate filling

Discharge after Check Disposal pathway

Waste water pathway no segregation

Process water

Sewer system

Alternate filling

Discharge after Check Disposal pathway

Waste water pathway no segregation

Process water

Figure 3.6: Self-contained buffer with alternative filling

Sewer system

Discharge after check Disposal pathway

Waste water pathway Process water

Process water

Sewer system

Waste water pathway

Disposal pathway

Discharge after check

Figure 3.7: Connected buffer, discontinuously filled

The continuously filled connected buffer can also be used as an equalisation or flow balancing tank (see Section 3.3.2.1). A control and alarm system ensures that the outlet to the WWTP is immediately closed in the case of an unusual event. The capacity of the tank must be sufficient to receive all the incoming waste water until the failure is remedied, so this system might be recommendable only for tributary streams. Before the operation can continue with the input of routine waste water, the tank has to be emptied.

Sewer system

Disposal pathway

Waste water pathway Process water

Sewer system

Disposal pathway

Waste water pathway Process water

Figure 3.8: Connected buffer, continuously filled

A further system (see Figure 3.9) is suited to receive and buffer leakage losses, with leakages collected in a separate sewer. Such a sewer is used as a drainage system for potentially polluted outdoor areas, e.g. with production installations or tank fields. The capacity of the tank matches the largest possible leakage loss and the amount of rainwater to be expected. This buffer system is applicable to installations with separated sewers for process water and drainage of risk areas.

Events that might influence the process water stream cannot be controlled. Its advantage is the ability to collect leakage losses in a concentrated state to enable recycling.

Sewer system

Discharge after check Disposal pathway

Waste water pathway Process water

Sewer system (only process water) Sewer system

Discharge after check Disposal pathway

Waste water pathway Process water

Sewer system (only process water)

Figure 3.9: Leakage buffer system

Applicability

The interim storage of contaminated rainwater requires segregation, which may not be applicable in the case of existing waste water collection systems.

Economics

The cost of buffering is primarily determined by the size of the tank, placement and the materials used. In general, one should consider a cost of EUR 100/m³ for a concrete tank, excluding accessories and placement. Buffer tanks made of other materials are normally more expensive, e.g. an enclosed 100 m³ polyester buffer tank with stirrer costs EUR 50000 [ 63, VITO 2010 ].

Driving force for implementation

Compliance with legal requirements to limit pollution to receiving waters is the main driving force for implementation of the technique.

Example plants

Virtually all WWTPs apply the technique. Examples of chemical sites using the technique are mentioned in Table 3.6.

Reference literature

[ 63, VITO 2010 ] [ 217, RP Freiburg 2009 ] [ 251, Ullmann's 2000 ]

3.3.2.3 End-of-pipe techniques 3.3.2.3.1 Overview

To introduce a logical order in the description of treatment techniques, the relationship between the pollutant and respective typical treatment technique is taken as the reference, as pointed out in Section 1.6.3.2 and illustrated in Figure 3.10. Table 1.1 lists the main waste water end-of-pipe treatment techniques and the pollutant(s) targeted for abatement by each technique. The details of the applicability of each technique are given in the specific sections as indicated in Table 1.1.

The first treatment step for waste water and rainwater – and often also the final step – is the separation of suspended solids and immiscible liquids (with regard to water) from the main water stream. Separation or clarification techniques are:

 gravity separation (grit separation – see Section 3.3.2.3.3.2, sedimentation – see Section 3.3.2.3.3.4, oil-water separation – see Section 3.3.2.3.3.8);

 flotation (see Section 3.3.2.3.3.5);

 filtration (filtration – see Section 3.3.2.3.3.6, membrane filtration – see Section 3.3.2.3.3.7).

These separation techniques are mainly used in combination with other operations, either as a first or a final clarification step. As a first step they, for example, protect other treatment facilities against damage, clogging or fouling by solids. As a final step, they remove solids formed during a preceding treatment operation or process, or remove oil before further biological treatment. They often follow treatment techniques applied to soluble pollutants, when these are transferred into solids. Examples are given later in this chapter.