| Summary: | Water treatment residues (WTRs) are the by-product from the production of potable water.
They consist mainly of the precipitated hydrous oxides of the treatment chemicals, and
materials removed from the raw water. This study investigated the range of treatment
processes and residues produced in South Africa, and two WTRs were selected for testing on
selected soils and mine materials.
A questionnaire was developed and sent to water treatment authorities across South Africa.
Information on the treatment chemicals, dosages, volumes and current disposal practices, and
a sample of WTR from each treatment plant were requested. Eleven, of 21 authorities,
returned completed questionnaires, representing 37 water treatment facilities. Organic
polymers were the most commonly used treatment chemical, with most plants also using lime.
Other less frequently used chemicals and additives were Alz(S04)3.14I-hO, Fe2(S04)3, FeC!),
sodium aluminate, activated silica, activated charcoal, CO2 and bentonite. Information given
regarding residue thickening and disposal was poor. Samples from Rand Water, Umgeni
Water (Midmar), Midvaal Water Company, Amatola Water and Cape Metropolitan Council
(Faure) were received or collected. An additional sample from Faure was also received,
representing a change in the treatment process. These samples were analysed for a range of
chemical and physical characteristics. These analyses showed that the WTRs had the potential
to supply some plant nutrients (Ca, Mg, Fe, S) but that metal toxicity may be a problem, in
particular Mn in the Faure WTR, and that P adsorption may be severe. The samples selected
to test the potential for land disposal were from Rand Water and Faure.
A pot experiment tested the growth of Eragrostis tefJ, Cenchrus ciliaris and Digitaria
eriantha in mixtures of Rand WTR and material from a coal mine i.e., a sandy soil material,
spoil material and coal combustion ash, at rates of 0, 50, 100, 200 and 400 g kg" with a
uniform fertiliser treatment applied to all mixtures. The grass was harvested on three
occasions and the mean total yield (dry mass) determined, as well as nutrient uptake. The pots
were leached after each harvest and the pH and electrical conductivity determined. The soil,
spoil and ash were characterised and pH, EC and water retention characteristics of the
mixtures determined. Growth of the grasses in the ash treatments was poor and these were
terminated. Eragrostis tefJ grown in the soil showed a decrease in mean total yield with
increasing WTR application rate, but yield was good up to the 200 g kg" treatment at the first harvest, declining substantially by the second harvest. In general C. ciliaris and D. eriantha
grown in the soil showed a decrease in mean total yield for all harvests with increasing WTR
application. The yield of E. /ejJ, grown in the spoil, increased up to 100 g kg,l WTR
addition, but decreased thereafter. Digitaria eriantha showed a decrease in yield, and C.
ciliaris an increase, with increasing WTR application rate , but for all treatments the
differences were non-significant. The pH and EC of the leachates generally increased with
increasing WTR addition. The concentration of nutrients in the grasses did not indicate any
deficiencies or toxicities.
As the growth of grass was poor in the ash treatments, another pot experiment was established
to test the growth of two creeping grass species grown in the Rand WTR as a cover over the
ash material. Cynodon dactylon and Stenotaphrum secundatum were grown in 20, 40 and 60
mm layers of Rand WTR, with and without a fertiliser treatment. Both species performed best
in the 60 mm layer with fertiliser, and C. dactylon performed better than S. secundatum. The
former species was more tolerant of the high pH, but both have potential as cover vegetation
on the ash dumps when these are covered with Rand WTR.
A further glasshouse study investigated the effect of Faure WTR mixed with a nutrient poor
sandy soil on the nutrient uptake and seed yield of common dry beans (Phaseolus vulgaris).
The WTR was added to the soil at 0, 50, 100, 200 and 400 g kg" each with five levels of
fertiliser (0, 25, 50, 100 (recommended optimum) and 150 %). Bean pods were harvested
once the plants had senesced. The number of pods and mass and number of seeds per
treatment were determined. The seeds were analysed for nutrient uptake. Interveinal chlorosis
and necrotic lesions were evident on cotylendonous and new leaves in the WTR treated soils,
the severity of the symptoms increasing with increasing rate of WTR. Additional pots were
established at the 400 g kg" rate (without fertiliser) and leaf material collected for chemical
analysis. This showed that Mn toxicity was the cause, with leaf concentrations about 12 times
the recommended 100 mg kg" upper limit. However, mass of bean seed was highest in the
400 g kg" Faure WTR treatment with 150 % fertiliser. Nutrient translocation to the seed
seemed to be relatively consistent regardless of treatment, with little accumulation ofMn.
The data collected illustrated the range of conditions and types of WTRs produced in South
Africa, and that in some instances these residues have favourable characteristics for land
application. The use of the Rand WTR showed that it could be applied to the spoil medium at
relatively high concentrations without severely negatively impacting on grass growth, but more caution should be used when applying this material to the soil medium. While the grass
did not grow in the ash treatments, it would seem that with suitable species the Rand WTR
could be beneficially applied to ash material as a cover layer. The use of the Faure WTR on a
sandy soil seemed to potentially improve the yield of the indicator crop, but caution should be
exercised due to the possibility of Mn toxicity. The use of additional fertiliser would seem to
be essential. Further research would require that field scale investigation of both WTRs be
conducted, as well as further studies of applicat ion rates and techniques in laboratory and
glasshouse investigations. === Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2003.
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