Characterisation of wastewater for modelling of wastewater treatment plants receiving industrial effluent

Bio-process modelling is increasingly used in design, modification and troubleshooting of wastewater treatment plants (WWTPs). Characterisation of the influent wastewater to a WWTP is an important part of developing such a model. The characterisation required for modelling is more detailed than that routinely employed for monitoring WWTP operation. Wastewater characteristics depend on the sources within the catchment served by a particular WWTP, and the presence of industrial effluents can cause the wastewater to be significantly different from purely domestic effluent. eThekwini’s wastewater treatment system is one of those most affected by industrial effluents in South Africa. Where industrial pollutants cause particular problems, additional measurements, beyond those standardly applied in sewage treatment, are required. Since influent characteristics vary from one catchment to another, this paper presents and compares results of influent wastewater characterisation from three municipal WWTPs, two of which are operated by eThekwini Water Services, which receive a combination of industrial and domestic wastewater. The paper also presents efforts to fractionate the influent COD of another WWTP run by eThekwini municipality and receiving a complex mixture of industrial and domestic effluent. The influent characterisation involves the determination of the volumes and concentrations of the carbonaceous, nitrogenous and phosphorus fractions in the wastewater, as well as other constituents present in the wastewater. This paper focuses on the carbonaceous fraction in the wastewater.Keywords: influent characterisation, industrial effluent, process modellin

Modelling the filling rate of pit latrines

Excreta (faeces and urine) that are deposited into a pit latrine are subject to biodegradation, which substantially reduces the volume that remains. On the other hand, other matter that is not biodegradable usually finds itsway into pit latrines. The net filling rate is thus dependent on both the  rate of addition of material and its composition. A simple material balance model is presented which represents the faecal sludge as a mixture of biodegradable organic material, un-biodegradable organic material and inorganic material. Measurements made on 2 pits in eThekwini, South Africa, were used to determine parameters for the model. Model predictions were then compared with data from 15 other pits in the same area and filling rate data from previous South African studies, which exhibit a 20th to 80th percentile range of 200 to 453 ℓ∙pit−1∙yr−1. These comparisons indicated that the pits studied exhibited relatively low filling rates resulting from orderly disposal practices. The average composition of the pit (COD, biodegradable material and inorganic fraction) changes with age, which will impact on any subsequent sludge treatment process. Pit filling rates are greatly affected by the disposal of solid waste in addition to the faecal material. For the pits studied, the model predicts that the filling time could have been extended from 15 years to over 25 years if all solid waste had been excluded from the pit.Keywords: Pit latrine, filling rate, biodegradation, solid waste disposa

Variation in VIP latrine sludge contents

This study investigated variations in the characteristics of the sludge content from different ventilated improved pit (VIP) latrines and variation in these characteristics at specific depths within each pit. Faecal sludge from 16 VIP latrines within the eThekwini Municipality was collected and laboratory characterisation including moisture content, total and volatile solids, chemical oxygen demand, and aerobic biodegradability was performed. Sludge samples were collected from 4 specific depths within each pit investigated. The laboratory characterisation performed showed that none of the VIP latrines investigated had the same sludge characteristics, and that within a pit sludge characteristics varied with increasing depth in the pit. This supports the motivating hypothesis that, depending on household habits and local environmental conditions, there should be considerable variation in the organic contents, moisture content, non-biodegradable content and microbial population between different pits. This variation with increasing depth within a pit is expected, since fresh material is constantly being added to the pit overlaying older material which might have undergone a certain degree of stabilisation

An investigation of the effect of pit latrine additives on VIP latrine sludge content under laboratory and field trials

Sludge content in VIP latrines is degraded mainly under anaerobic conditions and the process is relatively slow. At varying stages of digestion within pit latrines, sludge accumulates and odour and fly nuisance may occur which could pose risks to public health and the environment. Management of accumulated sludge in pit latrines has been a major problem facing a number of municipalities in South Africa and is also a global issue. Manufacturers of various commercial pit latrine additives claim that by addition of this product to pit content, accumulation rate and pit content volume can be reduced, thereby preventing the pit from ever reaching capacity. This paper presents a comprehensive study conducted to determine the effects of additives on pit contents under laboratory and field conditions. By conducting both laboratory and field trials, it was possible to identify whether there is any acceleration of mass or volume stabilisation as a result of additive addition, and whether any apparent effect is a result of biodegradation or of compaction. The results indicated that neither laboratory trials nor field trials provided any evidence that the use of pit additives has any beneficial effect on pit contents. The reasons why additives seem to not have any beneficial effects are also discussed.Keywords: additives, digestion, pit content, sludge, public health, VIP latrin

Integration of complete elemental mass-balanced stoichiometry and aqueous-phase chemistry for bioprocess modelling of liquid and solid waste treatment systems – Part 5: Ionic speciation

Where aqueous ionic chemistry is combined with biological chemistry in a bioprocess model, it is advantageous to deal with the very fast ionic reactions in an equilibrium sub-model, as was frequently mentioned in the preceding papers in this series. This last paper in the series presents details of how of such an equilibrium speciation sub-model can be implemented, based on well-known open-source aqueous chemistry models. Specific characteristics of the speciation calculations which can be exploited to reduce the computational burden are highlighted. The approach is illustrated using the ionic equilibrium sub-model of a plant-wide wastewater treatment model as an example

Integration of complete elemental mass-balanced stoichiometry and aqueous phase chemistry for bioprocess modelling of liquid and solid waste treatment systems – Part 4: Aligning the modelled and measured aqueous phases

Completely mass-balanced biological, physical and chemical process stoichiometry ensures that the CHONPS material and charge content entering and exiting bioprocess system models is conserved, which is a requirement for pH prediction in integrated physical, chemical and biological process models. Bioprocesses transform the material content from reactants to products, exchanging material between the aqueous, gaseous and solid phases, which cause pH changes in the aqueous phase. By measuring the material content of the aqueous phase, the progress of bioprocesses can be monitored. Alkalinity is an important aqueous-phase property that can be used to track aqueous-phase changes caused by physical, chemical and biological processes. Alkalinity is a stoichiometry property of the components in solution (i.e., a linear function of the amounts present). Its uptake from, and release to, the aqueous phase can both be modelled with bioprocess stoichiometry, and measured in physical bioprocess systems, and so aid in linking the modelled and measured aqueous-phase compositions. Changes in the concentrations of components containing the elements C, H, O, N, P and S result in changes in six weak acid/bases systems in the aqueous phase, all of which affect the total alkalinity. These are: inorganic carbon (IC), ortho-phosphate (OP), free and saline ammonia (FSA), volatile fatty acids (VFA), free and saline sulphide (FSS) and the water itself. Characterization of the aqueous phase to quantify the material content of the aqueous phase containing these six weak acid/base systems using the 5-point titration method is described. While several alkalinity titration based methods are available for anaerobic digestion bioprocess monitoring, only the 5-point titration is sufficiently accurate for aqueous-phase characterization to quantify the aqueous-material content for pH prediction in bioprocess models

Integration of complete elemental mass-balanced stoichiometry and aqueous-phase chemistry for bioprocess modelling of liquid and solid waste treatment systems – Part 1: The physico-chemical framework

Bioprocesses interact with the aqueous environment in which they take place. Currently integrated bioprocess and three-phase (aqueous–gas–solid) multiple strong and weak acid/base system models are being developed for a range of wastewater treatment applications, including anaerobic digestion, biological sulphate reduction, autotrophic denitrification, biological desulphurization and plant-wide wastewater treatment systems. In order to model, measure and control such integrated systems, a thorough understanding of the interaction between the bioprocesses and aqueous-phase multiple strong and weak acid/bases is required.  This first in a series of five papers sets out a conceptual framework and methodology for deriving bioprocess stoichiometric equations. It also introduces the relationship between alkalinity changes in bioprocesses and the underlying reaction stoichiometry, which is a key theme of the series. The second paper develops the stoichiometric equations for the main biological transformations that are important in wastewater treatment. The link between the modelling and measurement frameworks, which uses summary measures such as chemical oxygen demand (COD) and alkalinity, is described in the third and fourth papers. The fifth paper describes an equilibrium aquatic speciation algorithm which can be combined with bioprocess stoichiometry to provide integrated models of wastewater treatment processes

Predicting the efficiency of deposit removal during filter backwash

The long-term performance of granular media filters used in drinking water treatment is ultimately limited by the efficiency of the backwash process. This paper demonstrates that it is possible to develop quantitative predictions of backwash efficiency based on filter operating conditions. An experimental investigation into the effect of backwash rate, type of coagulant, degree of clogging and accumulation of residual deposits (not removed by backwash) on the efficiency of fluidised bed filter backwash in laboratory scale filters is described. A natural raw water was used and small variations in the raw water characteristics (manifested as variations in raw water turbidity, temperature, pH, rate of head loss development and turbidity removal efficiency) within each set of experiments appeared to affect the efficiency of backwash in addition to the parameters varied deliberately. Stepwise linear regression and statistical analysis of model significance were used to determine which of several possible filtration and backwash parameters were the best predictors of backwash performance. Backwash rate, filter run time, rate of head loss development and mass of residual deposits accumulated during previous runs were found to be the best predictors of backwash efficiency for any given filter cycle. Floc deposits appeared to become more difficult to remove the longer they remained in the filter, while rate of head loss development appeared to provide some indication of the strength of cohesive deposits for filter runs of similar length. The efficiency of detachment of freshly deposited floc appeared to increase as the mass of residual deposits and mud balls in the filter increased. The numerical correlations developed in this study are site specific but the methodology can be adapted to any filter operation and backwash regime