Members of the Kommetjie Residents’ & Ratepayers Association posed questions with regard to the waste water treatment and sewage plant at Wildevoelvlei, in the light of proposed developments in Kommetjie.
The reply below was sent by Peter King, specialist in Operations Support at the Wastewater Branch of the City of Cape Town.
The questions posed were:
What was the original design capacity of the waste water treatment and sewage plant?
What is the current operating capacity?
What quality standard is the effluent conforming to at present?
With the proposed increase in capacity is the current plant and control sytems adequate to maintain the legally required quality effluent standard likely to be in place within the next few years?
Peter King replied:
The first stage capacity of the Wildevoelvlei Wastewater Treatment Works was 7.2 ML/day and 36 000 population equivalent. This was extended in October 2000 to 14 Ml/day and 86 000 population equivalent. The works is currently treating a dry weather flow of 8 Ml/day and an average wet weather flow of 9.5 Ml/day. The organic loading is about 65 000 population equivalent.
An application for a Water Use Licence was made to the Dept of Water Affairs in November 2005. To date no licence has been forthcoming.
In the absence of prescribed effluent quality criteria, the treatment works is required to comply with the so-called 1984 standards. The 4 most important parameters being:
Suspended Solids – 25 mg/l; Chemical Oxygen Demand – 75 mg/l; Ammonia as N – 10 mg/l and Escherichia Coli – not exceeding 1 000 / 100 ml. The General Standard of 1984 does not specify a phosphate standard.
For the year ending May 2012; the compliance with these 4 parameters is as follows: Suspended Solids – 90.0%; Chemical Oxygen Demand – 93.9%; Ammonia – 96.0% and E. Coli – 100%. It is important to note that generally the works produces an effluent that falls WELL within these standards. The mean values over the same period are: Suspended Solids – 10 mg/l; Chemical Oxygen Demand – 46 mg/l, Ammonia – 1.9 mg/l and E Coli – 20 /100 ml. The latter is the 50th percentile value – this means that 50% of the results are equal to or less than this value.
For information and to stress the generally very high quality of the effluent, the 90th Percentile values are: Suspended Solids 19 mg/l; Chemical Oxygen Demand – 49 mg/l; Ammonia – 3.4 mg/l and E Coli – 80 m/100ml.
At the time of the design of the 2000 extension, the characteristics of the wastewater were such that the mathematical modeling of the treatment process indicated that biological excess phosphorus removal could take place. However, due to reasons that could not be quantified, the characteristics of the influent wastewater have changed so that only limited biological phosphorus removal is taking place. It is known that the phosphorus content of the final effluent does have an impact on the quality of the receiving water in Wildevoelvlei. With this in mind, the Wastewater Branch is able to report that the necessary equipment has been installed to permit chemical dosing of the effluent to reduce the phosphorus content of the effluent. The actual dosing has recently commenced. The next stage is to vary the dosage rate to obtain the required level of phosphorus reduction in the final effluent.
Considering the need to possibly extend the works in the future, it is noted above that some 5 ML/day and some 20 000 population equivalent capacity is available. Some of this unused capacity has been allocated to developments either under construction or in the initiation phase.
Several of the City’s wastewater treatment works where Water Licences have been issued are required to produce an effluent with an Ammonia content of 2 mg/l. As the Wildevoelvlei works is currently producing an effluent Ammonia close to this value, it is unlikely that large scale extensions will be likely. It may be necessary to downgrade the existing treatment capacity by a small amount.
As indicated above, chemical dosing of the effluent has begun. A stricter phosphorus standard than that anticipated, would require a larger chemical dosage with its concomitant increase in operating costs.
It is trusted that the information provided above fulfills your requirement.
You are welcome to contact the undersigned should you require more information. Perhaps, the reason for your request would enable me to be more specific in my reply.
P B KING
tel: 021 487 2603
fax: 021 423 9540 OR 0865 761 083
cell: 084 9099 354