Nitrate-nitrogen removal with small-scale reverse osmosis, electrodialysis and ion-exchange units in rural areas

Jj Schoeman

doi:10.4314/wsa.v35i5.49198

Abstract

The nitrate-nitrogen concentration in water supplied to clinics in Limpopo Province is too high to be fit for human consumption (35 to 75 mg/l NO 3 -N). Therefore, small-scale technologies (reverse osmosis, ion-exchange and electrodialysis) were evaluated for nitrate-nitrogen removal to make the water potable (< 10 mg/l NO3-N). It was found that the reverse osmosis process should function well for nitrate-nitrogen removal. Nitrate-nitrogen could be reduced from a concentration of 35 to 43 mg/l in 1 case to a concentration of between 1.4 and 5.5 mg/l in the treated water. In another case it could be reduced from 54 to 72 mg/l to 12 to 17 mg/l in the treated water. The water was also effectively desalinated. The ionexchange process could also reduce the nitrate-nitrogen concentration to less than 10 mg/l in the treated water. However, the water could not be efficiently desalinated and the process should function better when the level of total dissolved solids in the feed is not very high. The electrodialysis process should also function well for nitrate-nitrogen and salinity removal. However, the electrodialysis process is more complicated to operate. The reverse osmosis and ion-exchange processes are therefore suggested for nitrate-nitrogen removal at clinics. Capital costs for small-scale reverse osmosis and ion-exchange units are estimated at ZAR7 000 and ZAR10 000, respectively. Operational costs for reverse osmosis and ion-exchange are estimated at ZAR3.16/m 3 and ZAR3.60/m 3 of treated water, respectively.

Highlights

Many borehole waters in rural areas in South Africa are not fit for human consumption because the nitrate-nitrogen (>6 mg/l), fluoride (>1 mg/l) and salinity (>1 000 mg/l) concentrations are too high (Schoeman and Steyn, 2000)
Reverse osmosis (RO) test results on tap water The RO unit was first tested on tap water prior to installation at the clinic
The results showed that there was a decline in the percentage nitrate-nitrogen removal from the beginning to the end of the run

Summary

Introduction

Many borehole waters in rural areas in South Africa are not fit for human consumption because the nitrate-nitrogen (>6 mg/l), fluoride (>1 mg/l) and salinity (>1 000 mg/l) concentrations are too high (Schoeman and Steyn, 2000). High nitrate-nitrogen concentrations in drinking water can cause an illness called methaemoglobinaemia or ‘Blue Baby Syndrome’ in small children This happens when the nitrate is reduced to nitrite in the gastrointestinal tract and the nitrite reacts directly with haemoglobin in the bloodstream to produce methaemoglobin with consequent impairment of oxygen transportation. IX and ED can remove nitrate-nitrogen very effectively from contaminated waters (Tisseau, 1998; Kesore et al, 1997; Lauch and Guter, 1986) The control of these processes in rural areas should be easier than that of a biological process. It is believed that high nitrate concentrations in borehole waters in rural areas originate from natural organic matter in soil and from pit latrines in the vicinity of boreholes. Water desalination would be required to reduce water quality to potable standards

Methods

Results and discussion

Summary and conclusions