Showing papers in "Water SA in 1996"

Waterborne diseases: Update on water quality assessment and control

Abstract: Water-borne diseases are the most important concern about the quality of water. The pathogens involved include a wide variety of viruses, bacteria and protozoan parasites. Due to differences in size, structure, composition and excretion by humans and animals, their incidence and behaviour in water environments differ. This constitutes difficult challenges for testing the safety of water and the efficiency of treatment processes. Further complications are that many water-borne pathogens, notably the great majority of viruses as well as protozoan cysts and oocysts, are not readily detectable. In addition, the prevalence of various water-borne pathogens changes as selective pressures change. In view of the diverse and variable goalposts, new epidemiological data, and progress in technology and expertise, the methods and strategies for quality monitoring and control of water-borne diseases are continually being revised and updated. This paper reviews the latest approaches to water quality monitoring using indicators of human and animal faecal pollution, and new methods for the detection of viruses. The importance of simple, economic and rapid methods for high frequency basic monitoring of water quality and the efficiency of treatment systems is emphasised Reference is made to the fundamental need for microbiological quality data in the management of national and regional water resources and supplies.

Removal of lead ions from aqueous solution using sphagnum moss peat as adsorbent

Abstract: The use of sphagnum moss peat for the removal of lead from aqueous solution has been studied in batch experiments. Investigations include the effect of pH and temperature, and kinetics of adsorption. The adsorption equilibria data were found to follow Langmuir models. It was found that the efficiency of lead removal using sphagnum moss peat as adsorbent depended very little either on the reaction temperatures (10 to 40°C) or on the initial pH values (4.0 to 6.0). Kinetic data suggested that the adsorption process was endothermic and pore diffusion was not the only rate-determining step.

Treatment of exhausted reactive dyebath effluent using anaerobic digestion : Laboratory and full-scale trials

Abstract: Reactive dyes are difficult to remove from textile waste water due to their solubility and they pass through conventional aerobic biological sewage treatment systems and enter the receiving water body. Investigations into the use of anaerobic digestion to decolourise reactive azo dyes have been successful on a laboratory scale and the investigation was extended to full-scale trials. Exhausted reactive dyebath effluent (3 kl/d) was discharged into a primary digester (1.34 Ml) on weekdays for a 151-d period. On average, 48 kl/d of sludge was fed to the experimental and control digesters. The overflow was monitored for colour, sodium and sulphide concentrations. A laboratory digester was also set up to simulate the full-scale conditions but was operated at twice the exhausted dyebath loading recipe. No visual difference in colour was noted between the overflow of the primary or laboratory digester and the control digester, but elevated levels of sodium and sulphide were obtained due to the high concentration of sodium sulphate used in the reactive dyeing process. The laboratory digester became unstable at sulphide concentrations of 400 mg/l. However, the sulphide concentrations in the primary digester never increased such that it threatened digester stability.

Occurrence of metals in waters : an overview

Abstract: Presence of metals in natural, drinking and waste waters can imply two types of circumstances (depending on concentration and specific metal) : firstly, certain positive effects, especially, when the metals present in drinking water are essential for human life (e.g. Mo and Zn) ; secondly, some negative and toxicologically undesirable effects for both human consumption and the general environment (i.e. Cd, Hg). This paper gives an overview of outstanding aspects related to the chemical behaviour, occurrence, physiology and toxicology of the 25 metals most frequently found in waters : Aluminium, antimony, arsenic, barium, beryllium, boron, cadmium, calcium, chromium, cobalt, copper, iron, lead, lithium, magnesium, manganese, mercury, molybdenum, nickel, potassium, sodium, selenium, silver, tin and zinc. On the other hand, the maximum concentrations of metals in natural, drinking waters and waste waters are listed according to the actual Spanish regulations. Finally, references to the maximum levels of metals in drinking waters established by a recent proposal of Directive in the European Union, as well as the latest WHO's guideline values for metals have been also considered.

Assessment of ecosystem changes in response to freshwater inflow of the Kromme River Estuary, St. Francis Bay, South Africa : A network analysis approach

Abstract: We prevent results on changes which occurred at the ecosystem level over a period of 10 years in the relatively pristine Kromme River estuary, St. Francis Bay. South Africa. Carbon flow models were constructed for the estuary based on data collected before 1984 and again during the period 1988 to 1992. The first data set was collected during a period of regular and substantial freshwater inflow (average of about 117 x 10 6 (m 3 .a -1 ), and the second set after the impoudment of the river in 1984 and the resultant decrease in freshwater inflow to less than 2x 10 6 m 3 per annum. The salinity regime changed from one with gradient (35 to 15) to a homogenous one where the salinity remained virtually constant at 35 and higher since the construction of the dam in 1984. The flows models were analysed and global system properties, such as the total system throughput, the magnitutde and structure of cycling, development capacity, ascendancy and relative ascendancy, were used to assess whether the system has undergone change due to reduced freshwater inflow rates since 1984. Results showed that while some of the biotic compartments increased in biomass and abundance, others declined. Based on the results we conclude that the estuary has changed from a plankton-dominated system to one dominated by submerged benthic vegetation and macrobenthic invertebrates, in response to reduced freshwater inflows.

Removal of hexavalent chromium from aqueous solutions by granular activated carbon

Abstract: Batch adsorption trials were used to assess the effectiveness of using activated carbon as a sorbent for the removal of hexavalent chromium from solution. The results show that, although the pH for maximum efficiency varied slightly with the concentration of Cr(VI) being applied, the most effective pH was in the range 2.5 to 3.0. Based on linear regression analysis, the data obtained from the batch studies showed a good compliance with both the Langmuir and the Freundlich equations. The values obtained for the isotherm constants showed that the maximum adsorption capacity, X m , was 145 mg/g. An examination of the adsorption kinetics was also made and an analysis of the data shows that pore diffusion is not the only factor governing the sorption rate.

Review of seasonal forecasting techniques and their applicability to Southern Africa

Abstract: The development of a seasonal rainfall forecasting capability has recently become a priority of many research organisations in Southem Africa, but the methodologies used are still at an early stage of development. In other areas, high forecast skills are generally associated with tropical atmospheric variability, largely because of a thermally direct response of the tropical atmosphere to oceanic heat anomalies. Over South Africa, most current forecast skill relates to rainfall variability attributable to the tropical atmospheric circulation, including El Nino-Southern Oscillation (ENSO)-related anomalies. Consequently, highest forecastability exists in the summer rainfall region during the peak rainfall months, December to February, and is particularly high in areas that are strongly affected by ENSO activity. The extratropical atmosphere has an important influence on the rainfall of the region during the first half of the summer season, when forecast skill is relatively low. Occasionally, the extratropical atmosphere also remains dominant during the peak summer months, resulting in a poor forecast for that season. Consequently, an improved understanding of the response of the temperate atmosphere to tropical anomalies and internal blocking should result in considerably improved skill for seasonal forecasts throughout the summer season.

Anaerobic treatment of seafood processing waste waters in an industrial anaerobic pilot plant.

Abstract: Fish and shellfish canning industries produce waste waters whose characteristics depend upon the raw material processed which, in turn, varies throughout the year. Some production lines operate simultaneously, although it is possible to segregate or combine streams in order to optimise the treatment process. The main streams, produced in the cooking of mussel or tuna and in the manufacture of fish-meal, were treated individually or in combination at an industrial pilot plant, with an anaerobic central activity digester (CAD) of 15 m 3 , for two years. The most noticeable characteristics of wastes are their high organic load (COD 20 to 90 g/l) and the salinity (up to 14 g/l of Cl - ). Another problem is the high ammonia content, up to 4.5 g/l, produced after the degradation of proteins. A strategy for adapting sludges to the salinity and to the ammonia content was followed and specific methanogenic activities of 0.7 kg COD/kg VSS.d were achieved, with chloride, sodium and ammonia concentrations of up to 15.5, 9.7 and 3.5 g/l, respectively. COD reductions, applied OLR and HRT ranged between 70 and 90%, 5 to 6 kg COD/m 3 .d and 4.5 to 5 d, respectively. During the entire experimental period, nutrients addition was not necessary and pH remained neutral due to the high buffering capacity of the process (3 to 4 g CaCO 3 /l. Sudden changes in the influent composition did not affect the stability of the process, except when high suspended solids mixtures were treated. The biomass content in the digester varied around 11 g VSS/l and the mean specific methane production was 301 l/kg COD removed (15°C, 1 atm), with a biogas content of 60 to 65%. Hydrogen sulphide in biogas ranged from I to 4%. The mean anaerobic biodegradability was calculated for the main influents treated, being 84.9% for tuna-cooking effluents, 92.7% for mussel-cooking effluents, 79.4% for mixtures of tuna and mussel effluents and 71.4% for the tuna/mussel/fish-meal mixtures.

A quantitative assessment of the biotic integrity of the Okavango River, Namibia, based on fish

Abstract: This is the first calculation of an index of biotic integrity (IBI) for the Okavango River to quantitatively assess the biological status of the river. The assessment indicated a definite degradation of the biotic component of the Okavango River. Furthermore isolated problem areas were also identified such as habitat and trophic level degradation, increased pollution and reduced fish stocks at several localities along the river. Several measures were proposed to counteract this trend to ensure a sustainable fisheries component of the Okavango River in future.

Three-dimensional kinematic trajectory modelling of water vapour transport over Southern Africa

Abstract: Back and forward kinematic trajectory modelling has been undertaken for rain and no-rain days over the central interior of South Africa in mid-summer. No-rain days (rain days) are shown to be characterised by dry (moist)south-westerly (northerly to north-easterly) flow originating over the South Atlantic (tropical Indian) Ocean. Air parcels for tropical-temperate troughs originate over the tropical Indian Ocean and trace south and south-eastwards across Southern Africa, corresponding closely to the position of the trough-associated cloud band. Trajectory modelling of a cut-off low pressure system reveals the presence and interaction of a cold. dry, descending conveyor from the south and a warm, moist, ascending conveyor from the north.

An evaluation of sorbitol-fermenting bifidobacteria as specific indicators of human faecal pollution of environmental water

Abstract: The value of selected indicators for the assessment of faecal pollution, as well as to distinguish whether the pollution is of animal or human origin, was investigated. Faecal coliform bacteria, faecal streptococci and sorbitol- fermenting bifidobacteria were included as indicator organisms. Comparative tests were carried out on samples collected from a stream and river exposed to predominantly faecal pollution of domestic animal origin. Water from the same stream and river was also tested after downstream exposure to runoff from a low socio-economic informed settlement with limited sanitation. Samples were collected from perennial flow during the dry season and from storm-water runoff after thunder showers. Sorbitol-fermenting bifidobacteria were found to be reliable indicators of human faecal pollution. The ratio of faecal coliforms to faecal streptococci was in the order of 3.5 to 4.7 immediately after heavy exposure to faecal pollution of human origin. This ratio may distinguish between pollution of human and animal origin under certain conditions but is not a reliable indication of pollution origin. The results show that runoff from the informal settlement constituted a major source of human faecal pollution for a river used as a downstream source of water for human consumption. It further showed that faecal pollution of human and animal origin can reliably be distinguished by means of appropriate combinations of indicators which may include sorbitol-fermenting bifidobacteria.

Trophic niches in an Argentine pond as a way to assess functional relationships between fishes and other communities

Abstract: The goal of this study was the identification of trophic niches of several fish species and, consequently, the differentiation of habitats in terms of to diet components, seen as an indirect way of linking predators as a function of prey to the various limnological communities. Samples of digestive tracts from 11 fish species captured in 1992 at Guemes Pond (Azul. Argentina) were analysed. Fifhty-four food items were identified, which were grouped as 18 variables. Multivariate techniques were used to analyse the data. Four groups of samples were segregated on the hasis of their associated communities: . piscivorous species: Hoplias malabaricus and adults of Oligosarcus jenyasii; . phytoplankton-periphytophagous species: Briconamericus iheringhi, Cheirodon interruptus and Cyphocharux voga; . zooplanktophagous species: Odontesthes bonariensis; and . zoobenthiphagous species: Rhamdia sapo. Astyanax eigenmanniorum, juveniles of O. jenynsii, Corydoras paleatas (with incursions to the zooplankton), and Loricarichtus anus (with incursions to the phytoplton-periphyton). Cichlasoma facetum exhibits a general, opportunistic diet.

Desalination of calcium sulphate scaling mine water : Design and operation of the SPARRO process

Abstract: The South African mining industry discharges relatively small quantities of mine service water to the environment, but these effluents contribute substantially to the salt load of the receiving surface waters. The poor quality of mine service water also has significant cost implications on the mining operations. Of the two main types of mine service water encountered in the gold mining industry, the so-called calcium sulphate scaling type is found in the majority of cases. Preliminary testwork on this type of water using membrane desalination processes revealed that only the seeded reverse osmosis type of process showed promise. To overcome certain process problems and high operating costs with this system, a novel membrane desalination technique incorporating seeded technology, called the SPARRO (slurry precipitation and recycle reverse osmosis) process, was developed. The novel features of the new process included; a lower linear slurry velocity in the membrane tubes, a lower seed slurry concentration, a dual pumping arrangement to a tapered membrane stack, a smaller reactor and a modified seed crystal and brine blow-down system. Evaluation of the SPARRO process and its novel features, over a five-year period, confirmed its technical viability for desalinating calcium sulphate scaling mine water. The electrical power consumption of the process was approximately half that of previous designs, significantly improving its efficiency. Membrane performance was evaluated and was generally unsatisfactory with both fouling and hydrolysis dominating at times, although operating conditions for the membranes were not always ideal. The precise cause(s) for the membrane degradation was not established, but a mechanism for fouling (based upon the presence of turbidity in the mine water) and a hypothesis for a possible cause of hydrolysis (alluding to the presence of radionuclides in the mine water) were proposed. Product water from the SPARRO process has an estimated gross unit cost (including capital costs) of 383 c/m 3 (1994).

Filamentous organism bulking in nutrient removal activated sludge systems. Paper 2: Stimulation of the selector effect under aerobic conditions

Abstract: The Monod kinetic based selector theory for filamentous bulking control has guided research and application for the past two decades. This approach, which in its broadest sense recommends modification of system configuration or operation to introduce alternating feed starve conditions, is evaluated for control of the low F/M filament group which are ubiquitous and cause most of the bulking problems in N and N & P removal systems. In agreement with the literature, it was found that this approach induces in the sludge a rapid uptake of influent readily biodegradable COD and oxygen under batch fed conditions (selector effect), but it could not be demonstrated that this controlled low F/M filament proliferation because the control systems without the selector effect also did not bulk with low F/M filaments. However, the selector effect did control Sphaerotilus natans and Thiothrix sp. proliferation, the former of which was found to grow in the laboratory systems as a result of seeding from influent feed line wall growths. Batch test results could be adequately interpreted with existing activated sludge kinetic models, and based on these, a design method for aerobic selectors is presented. A selector designed with this method is shown to induce a selector effect and control filaments S. natans and Thiothrix sp.

Willingness to pay for water : The international debates

Abstract: The objective in this paperis to furnish a critical analysis of international research concerning the question of willingness to pay for water. The majority of existing international studies derives from research work either sponsored by or linked to the World Bank or other development agencies. The paper is divided into four sections of material. First, the significance of understanding willingness to pay is emphasised, particularly in terms of the planning of water projects. In the second section, the focus turns to a review of the contributions and validity of rapid reconnaissance surveys concerning informal water vending. In the third section the methodology and findings are reviewed of the cutting edge of willingness to pay research, namely contingent valuation studies. The final section draws together key conclusions and findings.

Filamentous organism bulking in nutrient removal activated sludge systems. Paper 6: Review, evaluation and consolidation of results

Abstract: The finding that the selector effect did not control bulking by low food to micro-organism ratio (F/M) filaments, concluded this research direction that was considered to hold promise for controlling low F/M filament proliferation, and placed this research back into an exploratory phase. In this paper, the information collected so far in the research programme is evaluated together with that published in the literature, in order to delineate new research directions aimed at solving the low F/M filament bulking problem. In the conclusions, a framework is established that provided guidance for the subsequent research.

The use of Selenastrum capricornutum growth potential as a measure of toxicity of a few selected compounds

Abstract: Algal growth potential (AGP) assays were used to determine the possible toxicity of a few selected compounds. The conventional EC 50 value was used to indicate toxicity. Four different parameters were used to determine algal growth rates in the presence of potentially toxic compounds, namely, cell numbers, dry mass, chlorophyll a measured fluorometrically and spectrophotometrically. Cu, Cd and atrazine were highly toxic, whilst Hg, phenol and gusathion showed little or no toxicity. The results showed that the time of exposure was very important, where on the one hand the algae adapted to the toxin rendering less toxic results, or long exposures indicating high toxicity where this was not apparent in the short exposures. The depletion of nutrients or the inability to distinguish between living and dead cells during long-term tests influenced the results, which made interpretations difficult. We recommend short-term tests and the selection of an appropriate growth parameter of which chlorophyll fluorescence gave promising results.

Metal concentrations in liver, kidney, bone and blood of three species of birds from a metal-polluted wetland

Abstract: The concentrations of cadmium, copper, nickel and lead in the liver, kidney, bone and blood of the herbivorous Redknobbed Coot Fulica cristata, the piscivorous Reed CormorantPhalacrocorax africonus and the omnivorous Sacred Ibis Threskiornis aethiopicus were determined. Specimens were collected from the Natalspruit wetland. This wetland area is polluted by these metals. The tissues were analysed for their metal contents and were processed according to standard analytical procedures. Cadmium levels were lowest in the four tissues analysed of all three species. Highest cadmium occurred in the kidneys of the ibis (3.4 μg/g), the bone of the cool (5.l μg/g) and the blood of the cormorant (4.7 μg/g). Copper (27 to 33 μg/g) was highest in the liver whilst nickel (11 to 36 μg/g) and lead (32 to 59 μg/g) occurred in the highest concentrations in the bone of all three bird species. Reed Cormorants generally exhibited the highest bone lead concentrations (59.0 μg/g). A high degree of variability in tissue metal concentrations was found among all three species. Except for liver, significant differences (p<0.05) were recorded for the four metals in all the other tissues of the three species. The research showed that these three bird species were able to accumulate these metals at abnormal concentrations with no apparent chronic or negative effect on their survival. In this context these birds therefore comply with one of the main criteria required for their potential as indicator organisms of metal pollution in the aquatic environment.

The role of zinc in magnetic and other physical water treatment methods for the prevention of scale

Abstract: line species released from the surface of physical water treatment devices into the feed water are shown to be primarily responsible for scale inhibition effects observed in three different types of physical water treatment (PWT) devices. The three types of PWT devices included units based on a magnetic field provided by a permanent magnet; a high frequency electric field; and a catalytic conversion process. Freshly released zinc from PWT devices was shown to have a marked effect on the induction period for CaCO, precipitation and on the crystal morphology of CaCO 2 . No measureable effect on the crystallisation reaction for calcium carbonate ascribable to the magnetic or electrical fields caused by the devices under investigation could be found.

Short-duration rainfall frequency model selection in Southern Africa

Abstract: Short-duration design rainfall estimates are vital in the design of hydraulic structures and for environmental management. The use of a digitised rainfall database is expected to improve these estimates in Southem Africa. In order to estimate design rainfall intensities, an appropriate probability distribution has to be chosen, which adequately fits the data. This paper investigates the use of goodness-of-fit tests to assess potential probability distributions and makes general recommendations of appropriate probability distributions for estimating short-duration design rainfall intensities in Southern Africa.

Checking anion-cation charge balance of water quality analyses: Limitations of the traditional method for non-potable waters

Abstract: The factors affecting the sound application of an anion-cation balance check to water quality analyses are investigated. It is shown that the carbonate contribution to the total negative charge (in meq/l) is satisfactorily calculated from the formula [HCO 3 ] = (Total Alkalinity)/50 below about pH 10 in samples in which carbonate is the only titratable component. It is also shown that ignoring the presence of titratable organic components may significantly affect this. It is shown that the contributions of H + and OH- to the total positive and negative charge respectively become important at an 0.1 meq/l level below about pH 4 and above about pH 10. Examples are presented that highlight the importance of knowing detailed speciation of ligands that protonate and of metal ions that hydrolyse. In each case, the contribution to the total negative or positive charge is significantly altered from that of the deprotonated ligand or unhydrolysed metal cation because the effective average charge on the predominant species is modified. Furthermore, strong complex formation between protonated ligands and metal cations and between hydrolysed metal cations and strongly binding ligands can significantly alter the charge that might be deduced from simplistic equilibrium distributions that ignore this binding.

Modelling of a relationship between phosphorus, pH, calcium and chlorophyll-a concentration

Abstract: The influence of algal growth on phosphate concentration in water was qualitatively simulated by means of a mathematical model. Phytoplankton, by carbon uptake through photosynthesis, decreases the carbonate concentration which results in equilibrium shifts through several connected chemical reactions that increase the supply of phosphorus to the phytoplankton. This internal cycling mechanism probably serves as a substantial source of P during the the development of an algal bloom. Based on these shifts, the developed mathematical model shows that, depending on the phytoplankton biomass, the phosphate (PO 4 -P) concentration in the water could increase, remain more or less the same, and/or decrease during an algal bloom. The proposed mathematical model suggests that calcium concentration, pH and chlorophyll-a concentration may play an important role in the solubility of phosphate in aquatic ecosystems.

Photochemical reaction of uranium with glucosamine, acetylglucosamine and related polymers : chitin and chitosan

Abstract: Uranyl ions can interact with amino-ligands and polymers such as chitin and chitosan by complexation and adsorption. This reaction can be affected by certain circumstances, such as a direct exposure to visible light, to give photoreactions. Among the main photoreduction mechanisms, sensitised photoactivation (SP) and charge transfer to mass (CTTM) are thought to explain the various responses obtained under several experimental conditions, where the nature of the ligand and the aerobic or anaerobic media are all changed. It seems that a preliminary complexation of uranium by ligand is required to optimise the reduction rate. The pH of the solution is significantly affected by the photoreaction, depending on the experimental procedure, and a precipitate appears after a duration. Elemental analysis and infra-red studies enable the structure of the precipitates to be determined as dihydrated uranate forms.

Filamentous organism bulking in nutrient removal activated sludge systems. Paper 4: System configurations and operating conditions to develop low F/M filament bulking sludges at laboratory-scale

Abstract: Intermittent aeration conditions (30% aerobic, 70% anoxic) in single-reactor continuously fed completely mixed systems sustain the growth ofthe low food to micro-organism ratio (F/M, long sludge age) filaments, Microthrix parvicella and Types 0092, 0041, 0675, 0914 and 1851, to give diluted sludge volume index (DSVI values of between 200 and 500 ml/g. Changing from intermittent to continuous aeration (DO, 2 to 4 mg O/l) ameliorates the low F/M filament bulking to give DSVI values as low as 60 ml/g in the absence of a selector effect.

Combined sorption technology of heavy metal regeneration from electroplating rinse waters

Abstract: A novel process arrangement is presented for treatment of electroplating effluents in order to minimise metal losses and prevent environmental pollution. The method comprises three processes: sorption of heavy metals from the catching bath solution, regeneration of sorbent and electroprecipitation of heavy metals in the electrolyser.

Human health aspects of certain metals in tissue of the African sharptooth catfish, Clarias gariepinus, kept in treated sewage effluent and the Krugersdrift Dam: Iron and manganese

Abstract: Health risks associated with the utilisation of waste water for fish production were studied by investigating the possible bioaccumulation of iron and manganese in the muscle tissue, kidney and liver of the African sharptooth catfish, Clarias gariepinus, kept in treated sewage effluent and in the Krugersdrift Dam, Bloemfontein, South Africa. Metal concentrations were also determined in the water and sediment of the mentioned localities. The water of natural water sources, such as the Krugersdrift Dam, was found to be more subject to changes in chemo-physical factors, e.g. pH and hardness, compared to that of treated sewage effluent. This finding correlates with the higher concentrations of Fe measured in the former-mentioned habitat during certain months of the year. The average wet mass concentrations of Fe and Mn in the muscle tissue for fish in treated sewage effluent (0.804 and 0.024 mg.g -1 respectively) and for fish in natural dam water (0.880 and 0.017 mg.g -1 respectively) were well below the recommended values set by health authorities for domestic water supplies. In contrast, the concentrations of these metals were noticeably higher in the liver and kidneys of catfish. As the latter concentrations approached the maximum permissible levels in the liver and kidneys of catfish, especially in the Krugersdrift Dam, these organs are not recommended for human consumption.

Seasonal variation of phytoplankton biomass in the middle Vaal River, South Africa

Abstract: The chlorophyll-a concentration in the Vaal River (at Balkfontein ; 1985 to 1989) was high (av. = 67 μg.l -1 ) and displayed great variation (8 to 360 μg.l -1 ). An increasing concentration trend of 20 μg.l -1 a -1 was shown. The hydrology, particularly episodic floods through inputs from summer rain, plays an important role not only in the chemistry, but also in the biology of the Vaal River. The chl-a concentration was usually the lowest after the summer rain period. It stayed low-for about two months and was then followed by a maximum concentration (bloom) in late winter to spring. The early bloom was dominated by diatoms, followed by a bloom usually dominated by green algae. The bloom was followed subsequently by a population crash. The enrichment of the river during floods by nitrogen (N) and phosphorus (P), usually leads to large phytoplankton blooms that occur approximately two to four months after floods. The average chl-a concentration in the Vaal River was statistically significant, correlated with the average total phosphorus (TP) concentration. Approximately 1 mg.l -1 increase in the average TP concentration will probably be associated by about 225 μg.l -1 increase in the average chl-a concentration. This aspect could make it fairly simple to predict and possibly control the standing crop in the Vaal River.

Filamentous organism bulking in nutrient removal activated sludge systems. Paper 5: Experimental examination of aerobic selectors in anoxic-aerobic systems

Abstract: Incorporation of correctly sized selector reactors at the head of an intermittently aerated (70% anoxic, 30% aerobic) system did not control the proliferation of low food to micro-organism ratio (F/M) filaments, viz. Microthrix parvicella, type 0092 and type 0914, even though the criteria indicating the induction of a selector effect had been met, i.e.: . removal of essentially all the influent readily biodegradable chemical oxygen demand (RBCOD); . the presence of a significant number of Zooglea colonies; and . a high initial oxygen utilisation rate (OUR) and RBCOD uptake rate under batch test conditions.

Consequences of an occasional secondary phosphorus release on enhanced biological phosphorus removal

Abstract: Laboratory experiments were performed to determine the conditions for an occasional secondary release of phosphates (release in the absence of any exogenous carbon input), and its consequences on the enhanced biological phosphorus removal process. Phosphorus accumulating sludge from two extended aeration waste-water plants was subjected, in an Erlenmeyer flask, to successive sequences of anaerobiosis, with or without the addition of raw water, and of aeration. Secondary release occurred between 1 and 3 h after decanting the sludge. It would appear that this phenomenon is linked to the low nitrate concentration in the aqueous phase of sludge (NO 3 -N < 0.5 mg/l). This release occurs at very low rates (0.2 and 0.4 mg P/g VSS.h) over an oxidation reduction potential range between +100 and +250 mV/NHE. This secondary release does not lead to any excess accumulation of phosphorus in the course of later aeration. It would not appear that the occasional secondary release affects the enhanced biological phosphorus removal process. On the one hand, the phosphorus released without COD is completely reabsorbed even after a long period of anaerobiosis (20 h), and, on the other hand, this secondary release has no effect on the further release in the presence of COD, nor on the good reabsorption of phosphorus.

A model for batch settling curve

Abstract: A model is developed for describing sludge blanket interface settling in a batch reactor. The sludge settling curve is considered as one entity, not an a conglomerate of several separate parts, and thus descrihed in one model. The model interprets sludge settling as a time-dependent phenomenon and it describes the settling process from the beginning to the end. Since the whole settling curve is modelled, sludge settleability is accurately assessed. Two parameters of the model can be easily estimated with a non-linear estimation method. The derivative of the model can be used for computing sludge blanket interface settling velocity. The agreement between the observed and the computed activated sludge settling curves shows that the proposed approach is justified.