Declaration 1 Plagiarism
6.3. Quality of treated wastewater discharged into the receiving environment per bulk
6.3.6. Residual chlorine
The free chlorine residual concentrations in the final treated effluents of the various WWTW plants are indicated in Table 6.3. The residual chlorine concentrations range from 0.1 to 0.5 mg/l. In terms of the general requirements of effluent from the treatment of wastewater, the National Water Act (Act 36 of 1998) (NWA) specifies concentrations not to exceed 0.25 mg/l (DWAF, 1998). The majority of the treatment plants have residual chorine concentrations below 0.25 mg/l, except in Tongaat, Genazzano, Verulam, Phoenix, Kingsburgh, Mpumalanga and uMkomaas wastewater treatment plants. The average residual chlorine of the treated wastewater in Table 6.3 were not recorded for uMhlanga, Dassenhoek, Central, kwaNdengezi, Southern and Cato Ridge. Total residual chloride is the total amount of chlorine present in a sample. This is the sum of the free chlorine residual and the combined available chlorine residual. Whereas, chlorine residual is the amount available for chlorine present in wastewater after a given contact time (20 minutes at peak flow; 30 minutes at average flow) under specific environmental conditions (USEPA, 2017). A free chlorine residual concentration less than 0.25 mg/l was observed at uMdloti, Northern, KwaMashu, New Germany, Hillcrest, uMmbilo, uMhlatuzana, Isipingo, Amanzimtoti, Fredville, Hammarsdale, Maghabeni and Craigeburn wastewater treatment plants. Also, free chlorine residual concentration less than 0.25 mg/l was observed at Tongaat, Genazzano, Verulam, Phoenix, Kingsburg, Mpumalanga and uMkomaas wastewater treatment plants. Therefore, it is suggested that the residual chlorine levels for the
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treated effluent be improved at non-complying WWWT before it is discharged into the receiving environment.
Table 6.3. Treated wastewater quality for the eThekwini District Municipality (Data from EWS, 2020)
Sample ID pH
Free Ammonia
mg/l
COD mg/l
EC
mS/m E.coli
Suspende d solids
mg/l
Residual chlorine
mg/l
Tongaat Works 7.2 25 106.2 76 16.8 21.6 0.3
Genazzano Works 7.3 26.2 127.6 79.5 >4851.8 57.2 0.3 Umdloti Works 7.4 8.7 59.2 85.3 >4845.4 7.2 0.2
Verulam Works 7.5 3.1 145 146 >12098 65 0.3
Umhlanga Works 7.4 20.6 84.2 75.7 >12334 11.2 -
Phoenix Works 7.3 18.3 52.3 72.5 0 8.3 0.3
Northern Works 6.9 6.2 62 77.9 0 15.5 0.2
KwaMashu Works 7.5 6.2 86.8 73.7 1221 33 0.1
New Germany
Works 7.3 23.3 154.8 68.5 5602 25.4 0.2
Hillcrest Works 7.4 6.5 142.8 57.1 492 54.3 0.2
Umbilo Works 7.4 19.5 243.8 76.1 140.5 118.5 0.2
uMhlatuzana
Works 7.5 21.5 85.3 86.5 81.3 6 0.2
Dassenhoek Works 6.6 0.1 23 82.2 0 16.5 -
KwaNdengezi
Works 6.9 7.1 125.3 58.7 - 44.5 -
Southern Works 5.5 8.9 4985 325.5 - 666.3 -
Isipingo Works 6.8 3.7 44.5 56.1 589.8 15 0.2
Amanzimtoti
Works 7.6 6.3 133.3 - 41 86 0.2
Kingsburgh Works 7.6 16.5 97.3 83.9 >12098 22 0.3 Fredville Works 7.5 6.1 44.8 40.9 >6270 2.5 0.2
Cato Ridge Works - - - - 164.5 - -
Hammarsdale
Works 7.8 25.3 79.8 248 40.3 18.3 0.1
Mpumalanga
Works 7.2 6 50.8 55.9 0 7.8 0.3
Magabheni Works 8 7.3 49.5 90.8 7.5 4.8 0.1
uMkomaas Works 7.2 0.5 38 98.5 2.5 2.5 0.5
Craigeburn 7.5 8.7 34.8 68.5 >6049 2 0.1
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6.4. Treated Water Quality requirements for MAR and implications on using treated waste water effluent for MAR in the study area
Hydrogeochemical processes between injected water, native groundwater and the subsoil occur when treated wastewater is injected into an aquifer (Willemsen, 2020). Under normal circumstances, the recharge water (treated wastewater) should not degrade the quality of the groundwater, nor impose any additional treatment requirement after pumping (Brissaud, 1999). Apart from those in Australia (NWQMS, 1995), regulations concerning aquifer recharge do not rely on the capability of the aquifer to remove pollutants to meet the water quality required within the aquifer (Brissaud, 1999). In practice, the recharge water reaching the saturated zone of the aquifer should have previously acquired the quality acceptable for drinking water (Brissaud, 1999).
If the aquifer recharge is direct, then the injected water should be potable and should, as a minimum requirement, meet the standards required by SANS 241:2015 or contained in the WHO guidelines for drinking water quality (WHO, 1996). Moreover, the injected water should be treated to prevent clogging around the injection wells; long-term health risks linked to mineral and organic trace elements; and the degradation of the aquifer (Brissaud, 1999).
Clogging is an operational problem largely related to the quality of the recharge water. The capacity of the aquifer to remove pollutants provides an additional barrier protecting the abstracted water quality (Brissaud, 1999).
According to Murray (2009), provisional recharge water guidelines for MAR are DOC = 4 mg/l, EC = 70 mS/m, chloride = 100 mg/l, sulphate = 50 mg/l and Nitrate & Nitrite as N, 5 mg/l. All the WWTW treated wastewater effluent in the greater Durban region except for the Craigeburn and Fredville works do not meet the SANS 241:2015 drinking water quality criteria for public water supply for free ammonia, COD and EC.
EC for all WWTWs except New Germany, Hillcrest, KwaNdengezi, Isipingo, Fredville, Mpumalanga and Craigeburn have an EC value of more than 70 mS/m. COD for Genazzano, Phoenix, Northern, Dassenhoek, Isipingo, Fredville, Mpumalanga, Magabheni, uMkomaas and Craigeburn do not exceed 75 mg/l. However, for the remaining WWTWs, it exceeded 75 mg/l.
Trace organic solutes, such as trihalomethanes and compounds containing two carbon atoms such as trichloroethylene, tetrachloroethylene and trichloroethane are attenuated by the aquifer material in approximate accord with the order of elution of these compounds in gas chromatography (Treweek, 1985). Following adsorption, these species are often biodegraded,
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as evidence by decreasing concentrations of COD and single carbon compounds are degraded at a rate ten times greater than the two carbon atoms (Treweek, 1985).
The wastewater effluent concentrations for free ammonia for Tongaat, Genazzano, Umhlanga, Phoenix, New Germany, Umbilo, Umhlatuzana, Kingsburgh and Hammarsdale exceed the 10 mg/l limit of the SANS 241:2015 standards. Through the proper operation of recharge basins, the ammonia concentration can be reduced to negligible levels, with some increase in the nitrate level. The needs for drinking and industrial process water can often be achieved by a well-run secondary treatment plant. With respect to surface spreading, both bacteria and viruses are removed in the soil column by filtration, adsorption and other natural means. In surface spreading, disinfection is not recommended since the danger from the formation of chlorinated organics may be greater than the changes from viruses and pathogens which might pass through the soil column.
The suitability of the treated wastewater generated at the various WWTWs suggests that further treatment is required before the treated wastewater is injected into the aquifers. As it stands, only the Craigeburn and Fredville works are compliant with the guidelines for drinking-water quality SANS 241:2015. The Craigeburn works has a volume of 1Ml/day of treated wastewater, while the Fredville produces 5 ML/day of treated waste water which complies for MAR application.
6.4 Identification of potential aquifers in the greater Durban Metropolitan region for