Comments on Zimbabwe Independent article on toxins in Harare Water

1. On 10 January, there was an article on the front page of the Zimbabwe Independent referencing a report prepared by a South African company, Nanotech. The article stated that there are deadly toxins in the treated water produced by Harare.
2. The article quoted from parts of NanoTech’s report and it is possible that the article may have mis-represented the report.
3. According to the article, NanoTech’s report claims improved treatment and reduced costs if chlorine dioxide is dosed to the water.
4. Chlorine dioxide is a recognised as a disinfectant that can be dosed prior to treatment to reduce tastes and odours in water. It is generated at the point of application. Generation and dosage of chlorine dioxide needs careful control if harmful chlorates are not to be present in the treated water.
5. Regarding Nanotech’s claims of benefits and cost savings from using chlorine dioxide, it should be noted that their business according to their web site is to sell chlorine dioxide dosing equipment and the chemicals from which chlorine dioxide is generated. Nanotech’s claims for chlorine dioxide should be confirmed by an independent consultant.
6. The most alarming aspect of the article is the claim that the treated water contains hepatotoxins and neurotoxins derived from algae. It is a fact that there are algae in the raw water, it is known that at times there are blue-green algae in the raw water that are capable of producing hepatotoxins and neurotoxins, and it is known that there are times when blue-green algae in Lake Chivero do produce these toxins. However, it is not clear from the article if the concentration of toxins was measured in the treated water or if it was assumed that there would be toxins in the treated water if there are algae in the raw water. It would be rash to assert that the treated water is dangerous without providing test results to establish the presence of toxins.
7. Powdered activated carbon is being dosed into the water at the head of the works to control tastes and odours. It should also remove algal toxins and a number of other substances of concern that may be present due to pollution of the lake with untreated sewage. The article is not clear whether NanoTech’s claim that the current treatment regime will not remove algal toxins is because there is a problem with activated carbon as a treatment or a problem with poor dosage of the activated carbon.
8. There is a need for an independent consultant to do tests to determine the best treatment regime for this very difficult water. Thereafter the chemical supply and dosage equipment can be competitively and transparently bid.

Source: Peter Morris

Consulting Engineer – BSc (Eng), MSc, DIC, PrEng(Zim), MZweIE, CEng, MICE