Investigating the occurrence of algae in the drinking water supply system of Harare, Zimbabwe

This study assessed the quality of drinking water in the water supply system for the City of Harare (Zimbabwe) by investigating the occurrence of algae and other water quality parameters that affect its growth. At Morton Jaffray Water Treatment Works (MJWTWs), samples were collected from the raw water inlet and treated water outlet points. In the distribution system, samples were collected from selected sites and grouped into four zones (1, 2, 3 and 4). The algal taxonomic groups that were found in both raw and treated water comprised of Cyanophyceae, Chlorophyceae, Bacillariophyceae, Euglenophyceae and Dinophyceae. It was found out that Microcystis aeruginosa followed by Anabaena were the most abundant species in both raw water and in the distribution system. All measured water quality parameters were within the Standards Association of Zimbabwe and WHO guideline values except for chlorine which had an average residual chlorine concentration that was lower than the WHO recommended lower value of 0.2 mg/L in parts of Zone 2. Morton Jaffray Treated Water does not completely remove algae, and there is a carry-over of algae into the distribution system. Boosting of chlorine is recommended for Zone 2 that had residual chlorine less than the WHO minimum threshold of 0.2 mg/L.


INTRODUCTION
Globally, water bodies such as lakes, reservoirs, streams and estuaries are facing the problem of eutrophication (Toor et al. ). This eutrophication is conducive for cyanobacterial bloom formation (Merel et al. ). Some cyanobacteria species, such as Microcystis, Anabaena, Nodularia, Nostoc and Oscillatoria, produce toxins during algal bloom periods (Krupadam et al. ). There are several health implications that are associated with algae toxins, for example neurotoxins cause tremors, convulsions, heavy breathing and dizziness (Piontek & Czyż ewska ).
Nevertheless, contaminated water is still used for potable uses due to the lack of better sources of water. Lake Chivero in Zimbabwe has been reported to be eutrophic for quite some time, and this has caused the occurrence of dense blooms of bluegreen algae (Ndebele & Magadza ). Hoko & Makado ()  This has led to user rejection resulting in poor relations with customers and low willingness to pay for the service (Dandadzi et al. ). Public perceptions of water quality ranged from unsafe to highly contaminated (Chirenda et al. ). This study was carried out in Harare in the period from May to June 2017 and investigated the occurrence of algae in the raw and treated water at MJWTWs as well as at selected points in the distribution system. The study also assessed the levels of key parameters that affect growth of algae which included pH, turbidity, total nitrogen, total phosphorus and free residual chlorine.

STUDY AREA
Harare is the capital city of Zimbabwe. Greater Harare is situated on Zimbabwe's Highveld between latitudes 17 40 0 S and 18 06 0 S and between longitudes 30 37 0 E and 31 16 0 E (World Bank ). Harare is the industrial hub and the seat of government and the commercial centre of Zimbabwe (World Bank ). Figure 1 shows the location of Harare and the specific study areas.
Background on water supply for the city of Harare Harare City Council supplies water to its residents and those of Chitungwiza, Norton, Ruwa and Epworth (Nhapi & Hoko ). Drinking water in Harare is produced from two water treatment plants, MJWTWs and Prince Edward Water Works (Muisa et al. ). Figure 2 shows the water supply infrastructure for Harare. There are approximately 192,000 consumer connections in Harare (Chisango ).
According to Ndunguru & Hoko (), the pipe age for Glen View is 34 and 47 years in Mabelreign. The age of all the other suburbs under study is estimated to be 40 years with some over 60 years. The pipe diameters in the Harare water distribution system range from 50 to 1,500 mm. The network has 6,000 km of water pipes (Chirenda et al. ). Pipe materials are steel, asbestos cement and unplasticised polyvinyl chloride (Ndunguru & Hoko ).

Selection of study area and sampling points
Sampling was done at MJWTWs for both raw and treated water. In the distribution system, sampling sites were

RESULTS
The results for water quality analysis of Morton Jaffray Raw Water (MJRW), Morton Jaffray Treated Water (MJTW) and the distribution system are shown in Table 1.
Physical and chemical water quality parameters for raw water and in the distribution system pH The pH values for all study sites are shown in Figure 5. The pH value of MJRW was 7.30-8.20 (mean 7.52). The pH value of MJTW ranged from 6.98 to 7.02 (average 7.0).
In Zone 1 (Kuwadzana 2 and Kuwadzana 5), the pH values were 6.97-7.02 (average 6.99). In Zone 2 (Glen View and Lochinvar), it was 6.96-7.01 (average 6.98). In Zone 3 (Waterfalls and Dzivarasekwa), this was 7.00-7.04 (average 7.02). In Zone 4 (Belgravia and Mabelreign), the pH value ranged from 6.93 to 7.09 (average was 6.98). In most distribution networks, the pH value of final treated water is in the range of 6.5-8.5 (Aghaarabi et al. ). The pH value in the Accra water distribution system ranged from 6.80 to 7.80 (Karikari & Ampofo ). The optimum pH value for algal growth is in the range of 6.5-8.5 (Giannuzzi et al. ), and thus, the pH value of the water was suitable for algal growth.
A Pearson correlation coefficient of 0.2 was found between pH and algal concentration, suggesting a weak relationship. There was no significant change in pH in the distribution system beyond the treatment plant. A one-way ANOVA at 5% confidence interval showed no significant differences (p ¼ 0.16) in pH between each zone and treated water for MJTW. It was found out that the value of pH of all treated water samples was within the SAZ () and WHO () pH limits of 6.5-8.5.

Turbidity
The variation of turbidity for all study sites is shown in

Total phosphorus
The variation in TP for all sampling points is presented in Figure 8. The TP value of MJRW ranged from 0.16 to    There were no significant differences in TP concentration between each zone and MJTW except Zone 4 (p ¼ 0.02). The high levels found could be attributed to pollution in the lake and intrusion of dirty water in the distribution system due to an ageing system.

Free residual chlorine
The trend of free residual chlorine for all study sites is shown in Figure 9.   Species and abundance of algae in raw water and the water distribution network

Species of algae
The algal community in both raw and treated water was composed of five taxonomic groups: Cyanophyceae (three species), Chlorophyceae (six species), Bacillariophyceae (two species), Euglenophyceae (two species) and Dinophyceae (two species) as presented in Table 2.

Abundance of algae
The concentration of algae for the different species for all sampling sites is shown in Table 3, and the total algal concentartion is shown in Figure 10. The algal concentration of MJRW was 1000-2740 cells/mL (average 1,533 cells/mL).  algal concentration in the distribution system, suggesting regrowth in the system. This could be due to the fact that free residual chlorine concentration in the distribution system was much lower than that at the treatment plant. Chlorine inhibits the growth of microorganisms (Aghaarabi et al. ).
Algae increased from the treatment plant to Zone 1. However, there was a larger increase in algal concentration from the plant to Zone 2. The concentration of algae in Zone 3 was lower than that in Zone 2, but Zone 4 had a higher concentration of algae than Zone 3.
Areas that had the highest concentration of free residual chlorine (Figure 9) had the least concentration of algae