Method and installation description
The aim of a buffer is to realise a consistent volume and possibly a consistent quality. It is implemented to allow further purification processes to run as effectively as possible. The buffer can be in-line or off-line. In-line buffering involves the entire quantity of wastewater flowing through the buffer tank. In off-line buffering, only part of the wastewater flows into the buffer tank at specific moments (e.g. to cope with accidents or very high volumes). In principle, the buffer tanks feature a level-measure (to determine the level when controlling volumes and generating alarms) and a mixer-stirrer to realise a good mix (or buffering).
Specific advantages and disadvantages
By buffering one is able to avoid or limit peaks in wastewater volume and concentration, whereby:
- Later purification techniques are better equipped to treat the wastewater; on the one hand, examples include dosing chemicals (see technical file W17), sedimentation (see technical files W27 and W28) or flotation (see technical file W33), which are all greatly determined by the influent volume and, on the other hand, examples include biological purification (see technical files Wx), which benefits greatly from a consistently quantitative and qualitative wastewater flow.
- It becomes possible to ‘bridge’ periods in which no waste water is discharged using wastewater from the buffer tank; this allows water purification to work continuously even though wastewater isn’t continuously discharged.
There are no, or hardly any, disadvantages when buffering wastewater. Though the following should be taken into account when designing a buffer tank:
- The cost price of the buffer tank; this can rise greatly in case of major fluctuations in concentration or volume, or if long periods must be bridged.
- Possible reactions that take place in the buffer tank if it has a long retention time.
Thus acidification reactions may take place in wastewater from a food company which has a particular fat content in the wastewater. Reactions in the wastewater (acidification, fermentation, etc.) may result in smell problems, foam production, precipitation, etc. For wastewater with intensive acidification, it is also important to select construction materials or coating that is able to resist this.
In off-line buffering the wastewater only flows into the buffer tank if particular conditions have been met, for example, for a peak volume or an abnormally high concentration of one or multiple contaminants. Wastewater purification plants can be very sensitive to volume fluctuations or variations in influent loads. However, the quality and quantity of industrial wastewater are rarely consistent because of the encountered batch processes, stasis during the night or the weekend and the start-up or termination of processes. That is why buffering is used in just about all purification processes and in just about all sectors. The following practical objectives can definitely be realised:
- Equalisation of the influent volume.
- Equalisation of the influent load.
- Temporary treatment of periodic discharges, such as reversing ion exchangers or sand filters.
- Temporary treatment of peak loads or difficult to process or toxic contaminants.
When dimensioning the buffer tank, the volume variations as well as the variations in concentration must be taken into account. Initially, these variations must be established using a comprehensive sampling and volume measurement campaign. This data can then be used to perform a statistical analysis or a simulation in order to determine the optimum buffer tank size. There are two important objectives when doing this: the hydraulic objective (i.e. levelling hydraulic peaks) and load buffering. Both are conflicting: Thus a buffer tank that has been hydraulically optimised will normally not be sufficient to deal with variations in influent concentrations. In order to deal with them, one must maintain a particular volume of wastewater in the tank, which is not always the case in optimised hydraulic buffering (tank becomes empty after a particular period). Simulations of the level and concentrations in the buffer tank may indicate the extent to which influent variations are levelled off at a particular tank volume.
The chemical and microbiological stability of the wastewater must certainly be taken into consideration.
No residual products are released.
The cost of buffering is primarily determined by the size of the tank, placement and the used materials. In general, one should consider a cost price of €100/m³ for a concrete tank, excluding accessories and placement.
Buffer tanks in other materials are normally a lot more expensive. For example, an enclosed 100m³ polyester buffer with mixer-stirrer, costs €50 000.
Level of automation
Very easy to automate.
- AEA Technology, Manual of Effluent Process Technology, Environmental & Process Engineering Department, Harwell (GB), 1991
- Baeyens J., Hosten L. and Van Vaerenbergh E., Wastewater purification, Environment Foundation - Kluwer Editorial, 1995
- Giesen A., Crystallisation process enables environmental friendly phosphate removal at low costs, in Environmental Technology, Vol. 20, Issue 7, pp 769-775, 1999
- VITO-SCT, revision of technical files WASS, 2009
Version February 2010