Valuing the subsurface pathogen treatment barrier in water recycling via aquifers for drinking supplies

A quantitative microbial risk assessment (QMRA) was performed at four managed aquifer recharge (MAR) sites (Australia, South Africa, Belgium, Mexico) where reclaimed wastewater and stormwater is recycled via aquifers for drinking water supplies, using the same risk-based approach that is used for public water supplies. For each of the sites, the aquifer treatment barrier was assessed for its log(10) removal capacity much like for other water treatment technologies. This information was then integrated into a broader risk assessment to determine the human health burden from the four MAR sites. For the Australian and South African cases, managing the aquifer treatment barrier was found to be critical for the schemes to have low risk. For the Belgian case study, the large treatment trains both in terms of pre- and post-aquifer recharge ensures that the risk is always low. In the Mexico case study, the risk was high due to the lack of pre-treatment and the low residence times of the recharge water in the aquifer. A further sensitivity analysis demonstrated that human health risk can be managed if aquifers are integrated into a treatment train to attenuate pathogens. However, reduction in human health disease burden (as measured in disability adjusted life years, DALYs) varied depending upon the number of pathogens in the recharge source water. The beta-Poisson dose response curve used for translating rotavirus and Cryptosporidium numbers into DALYs coupled with their slow environmental decay rates means poor quality injectant leads to aquifers having reduced value to reduce DALYs. For these systems, like the Mexican case study, longer residence times are required to meet their DALYs guideline for drinking water. Nevertheless the results showed that the risks from pathogens can still be reduced and recharging via an aquifer is safer than discharging directly into surface water bodies.

Environmental and economic performance assessment of the integrated urban wastewater system

In order to comply with the Water Framework Directive's requirement to reveal the major pressures and impacts on the receiving water at river basin level, the merits of a methodology that combines substance flow analysis and mass balances were evaluated with the aid of a case study. The river basin analysis consisted of the analysis of all individual municipal sewer catchments constituting the basin on a yearly time scale, and included the description of the main sewers and waste water treatment plants and their performance in environmental and economical terms. A wide set of indicators was evaluated. Uncertainties and information gaps arising from the study are described. The choice of the geographic scale seems a key factor in the evaluation. The case study indicates that such an evaluation is of great value for decision-makers in the perspective of the Water Framework Directive implementation, to highlight situations of weak or strong performance and to pinpoint information gaps requiring further research in order to take more informed decisions, to identify the main pressures on the environment and to plan more cost-effective measures.

The role of free water surface constructed wetlands as polishing step in municipal wastewater reclamation and reuse

In Europe, the last two decades witnessed growing water stress, both in terms of water scarcity and quality deterioration, which prompted many municipalities for a more efficient use of the water resources, including a more widespread acceptance of water reuse practices. Treatment technology encompasses a vast variety of options. Constructed wetlands are regarded as key elements in polishing conventionally treated wastewater for recreational and environmental applications. A survey was conducted to assess the performance of tertiary free water surface constructed wetlands in treating both key and emerging contaminant categories in the perspective of water reuse. A database was created with information concerning systems with emerging and free-floating macrophytes. The database includes results from both full- and pilot-scale systems, and considers a broad variety of operating conditions. This paper provides an overview of the treatment performances of the constructed wetlands in the database and discusses their significance in the optic of water reclamation and reuse practices.

The removal of pathogens in surface-flow constructed wetlands and its implications for water reuse

Microbiological quality represents the biggest concern to the reuse of treated wastewater. This paper reports and discusses the results of an international survey on the removal of indicators of microbiological contamination in surface-flow constructed wetlands. Constructed wetlands consistently provide a reduction of 90-99% (1-2 log-removal) in the concentration of indicators such as coliform bacteria and faecal streptococci. This removal is found in wetlands treating water from different types of pretreatment (primary sedimentation, activated sludge, trickling filter, maturation ponds). On the other hand, when the influent is of high microbiological quality, wetlands act as sources of pathogenic contamination. The final water quality, however, is still compatible with medium to no-contact recreational activities and other final water uses. High variability in the effluent quality and seasonality might limit the opportunities for reuse. The role of constructed wetlands in different treatment schemes and the remaining open questions concerning removal mechanisms and reference pathogens are discussed.

Assessment of WWTP design and upgrade options: balancing costs and risks of standards' exceedance

Numerical models can be used to evaluate design and upgrade scenarios of urban wastewater systems on the basis of their ecological consequences. The objective of this paper is to illustrate a systematic procedure of system design/upgrade. This procedure consists of the following steps: (1) data collection and data reconstruction; (2) model building and calibration; (3) evaluation of scenarios; (4) uncertainty assessment. In contrast to conventional practice, this approach allows to choose the most appropriate trade-off between cost of measures and risk of non-compliance with regulatory limits. An example of its application dealing with the assessment of WWTP design and upgrade options is provided. Results show that by reducing the tank volumes compared to conventional design procedures, costs can be reduced sensibly while the risk of not meeting legislative requirements are only slightly increased.

Estimating costs and benefits of advanced control for wastewater treatment plants--the MAgIC methodology

This paper discusses a methodology to estimate the costs and benefits of advanced control for wastewater treatment plants. The methodology has been applied to four wastewater treatment plants, representing four standard types of plants built in Flanders, Belgium. The paper outlines the methodology and illustrated results from one of the four design cases. General results are shown and contrasted with full-scale experience. The methodology appears to give realistic results and will be used for further refinement of default control algorithms for certain types of plants. A preliminary analysis indicates that on-line control can become cost-effective for plant sizes above 50,000 population equivalents.

Benchmarking of WWTP design by assessing costs, effluent quality and process variability

Process choice and dimensioning of WWTPs is a particularly sensitive step to cost-efficiently comply with regulatory standards. This step accounts only for a small fraction of the upfront costs, but it can lead to substantial savings. This paper illustrates the results of a systematic methodology to evaluate system design/upgrade options. In contrast to conventional practice, this approach allows the choice between the most appropriate trade-off between cost of measures and effluent quality, and to assess the reliability of a process layout. It is therefore a flexible instrument to cope with the flexibility and complexity of integrated water management regulations. Results show good agreement of the simulations with extensiv benchmarking studies on actual plants. For that reason, the suggested methodology can provide valuable support also to such practices.

Technical-economical evaluation of the operation of oxidation ditches

The optimisation of the economic performance is a central aspect in the management of wastewater treatment plants. A model-based procedure was developed that could provide a clearly enunciated and concise way of reporting to the stakeholders on how well the assets are performing and can perform, separating the firm inefficiency from the effect of the treatment. The applied modelling method is conventional considering current modelling research practice, but very good from practical point of view in that it is applicable utilising existing plant data, i.e. without the use of additional measuring campaigns. This paper illustrates the validity of the developed procedures through the evaluation of the performance of oxidation ditches.

Experience with phosphorus removal and sludge handling and disposal in Flanders

The way excess sludge must be disposed of is a key factor in the choice of the appropriate phosphorus removal technique at municipal wastewater treatment plants. In Europe the ongoing trend of tightening the sludge spreading rules called for a serious reduction of its agricultural utilisation and the expansion of the (co-)incineration disposal route, which led to a shift towards more sophisticated sludge handling techniques. This paper illustrates the impact of different sludge handling techniques on the performance of chemical and enhanced biological phosphorus removal at municipal WWTPs. The main conclusion is that although enhanced biological phosphorus removal is particularly sensitive to the problem of return liquors from sludge treatment processes indirect dewatering and anaerobic stabilisation cannot be discarded altogether when considering its implementation.

Biological solution to storm water?

Standard practice in Flanders is to limit the hydraulic capacity of sewage treatment works to 6Q14 (Q14= 1.7 dry weather flow Q(DWF)). A maximum of 3Q14 is treated biologically, while the excess flow undergoes only physical treatment in storm tanks. This practice has been challenged by a new high-flow activated sludge operation concept, consisting of the treatment of the full storm sewage flow in the biological train and of the use of the storm tanks as additional secondary clarifiers. After successful testing in two installations, 56 works of different sizes and types were switched to high-flow activated sludge operation from 1999 to 2002. This paper reports on progress and experiences gained since then. The analysis focuses on the parameters subject to regulatory discharge (BOD, COD, suspended solids, total nitrogen and total phosphorus) plus ammonia. Special attention is paid to the performance of the clarification and of the nitrification processes. The results indicate that high-flow biological treatment provides a substantial reduction in wet weather discharges while maintaining acceptable process operating conditions.

The inhibitory effects of heavy metals and organic compounds on the net maximum specific growth rate of the autotrophic biomass in activated sludge

A respirometry technique can be applied as an effective method to determine the net maximum specific growth rate of autotrophic biomass under both normal conditions and when inhibition occurs. The net maximum specific growth rate of uninhibited autotrophic biomass, expressed as (mu(A)-b(A)), is approximately 0.8 per day [Proceeding of the International Congress on CHISA, Prague, 2002, p. 1]. Several heavy metals and organic compounds have inhibitory effects. Copper (Cu(2+)) has stronger inhibitory effects than zinc (Zn(2+)), and inhibits the nitrification process by 50% at 0.08 mg/l [(mu(A)-b(A)) = 0.4 per day], while the same concentration of Zn(2+) establishes 12% inhibition only [(mu(A)-b(A)) = 0.75 per day]. Inhibition with Cu(2+) starts at concentrations above 0.05 mg/l, while this is above 0.3mg/l for Zn(2+). The inhibition of the nitrification process is complete at 1.2mg/l for both Cu(2+) and Zn(2+). Among the selected organic compounds tested n the experiments, the degree of inhibition decreases as follow: chlorobenzene>trichloroethylene (TCE)>phenol>ethylbenzene. Chlorobenzene already inhibits the autotrophic biomass at 0.25 mg/l. The nitrification process is totally inhibited by adding 0.75 mg/l of chlorobenzene. TCE has a less inhibitory effect on the nitrification process and 50% inhibition is noticed at 0.75 mg/l TCE. The nitrification process is totally inhibited at 1mg/l TCE. Phenol inhibits the nitrification for 50% at 3 mg/l. The inhibitory effect of phenol is almost constant in the range 4-10 mg/l and complete inhibition is reached at 50 mg/l. The inhibitory effect of ethylbenzene is 50% at 8 mg/l and the autotrophic biomass is totally inhibited at 50 mg/l. Experimental findings are compared with literature data, which generally and significantly overestimate the inhibition threshold concentrations.

Two years of full-scale storm operation experience

Current practice in Flanders is to limit the hydraulic capacity of the wastewater treatment plant to 6O14 (O14 = 1.7 dry weather flow O(DWF)). A maximum of 3O14 is treated in the biological system, the excess flow undergoing only physical treatment in the stormtank. This practice has been challenged by a new concept, consisting of the treatment of the full storm sewage flow in the biological train and of the use of the stormtank(s) as additional secondary clarifier(s). This paper reports on the long-term experience gained on a total of 12 full-scale plants. The analysis focused on the parameters subject to regulatory discharge (BOD, COD, suspended solids, total nitrogen and total phosphorus) plus ammonia. Special attention is paid to the performance of the clarification and of the nitrification processes. The significance of the proposed way of operation in attenuating the overall pollution impact on the receiving water body is shown.

A quantitative risk analysis tool for design/simulation of wastewater treatment plants

Uncertainty is a central concept in the decision-making process, especially when dealing with biological systems subject to large natural variations. In the design of activated sludge systems, a conventional approach in dealing with uncertainty is implicitly translating it into above-normal safety factors, which in some cases may even increase the capital investments by an order of magnitude. To obviate this problem, an alternative design approach explicitly incorporating uncertainty is herein proposed. A probabilistic Monte Carlo engine is coupled to deterministic wastewater treatment plant (WWTP) models. The paper provides a description of the approach and a demonstration of the general adequacy of the method. The procedure is examined in an upgrade of a conventional WWTP towards stricter effluent standards on nutrients. The results suggest that the procedure can support the decision-making process under uncertainty conditions and that it can enhance the likelihood of meeting effluent standards without entailing above-normal capital investments. The analysis led to reducing the capital investment by 43%, producing savings of more than 1.2 million euro.

Feasibility of automatic chemicals dosage control--a full-scale evaluation

This contribution discusses the feasibility of automatic control for chemicals dosing in activated sludge systems. The evaluation is made on the basis of a full-scale implementation at the Lommel WWTP (Belgium) of an on-line controlled dosage system for iron chloride and external carbon source. The control laws are very simple and allow intuitive adaptation by the plant operators. The control system results in a significant reduction of the chemicals dosage, better effluent results and a lower sludge production. The implementation is furthermore cost-efficient.

Impact of cold and dilute sewage on pre-fermentation--a case study

The municipal sewage treatment plant (STP) of the city of Ghent (Belgium) has to be retrofitted to a 43%-increase in the nitrogen treatment capacity and to phosphorus removal. Cold weather, dilute sewage and a critical COD over N ratio make the retrofit a challenge for full biological nutrient removal. The potential for fermentation of primary sludge to alter those critical feed sewage characteristics was experimentally evaluated. The idea was that the pinpoint introduction of fermentate could optimise the available reactors by achieving high-rate denitrification and enhanced biological phosphorus removal. The fermentation process was evaluated with a bench scale apparatus. At 20 degrees C (heated process), the hydrolysis yield--expressed in terms of soluble COD--varied from 11% to 24% of the total sludge COD. The fermentation yield expressed in VFA COD varied from 8% to 13% of the total sludge COD. The efficiency of heated fermentation of primary sludge was lower during cold and wet weather, due to the different sewage characteristics, as a result of extended dilution periods and low temperature. The raw sewage, the primary effluent and the fermentate were fractionated according to the requirements for the IAWQ Activated Sludge Model No. 2d. The results clearly show that fermentation in the sewer played an important role and temperature was the driving parameter for the characteristics of the dissolved COD. Instead, the weather flow conditions were the driving parameter for the characteristics of the suspended COD. The results of the detailed fractionation were used as background for process evaluation. The final scenario choice for the retrofit depends on a cost-efficiency calculation.

Renovation of the WWTP hove

The provision of reliable treatment has always been a priority in sanitary engineering. However, there is an ever-increasing pressure to meet desirable standards at reduced costs. This paper sets out a methodology to enhance the likelihood of a satisfactory allocation of the resources while providing reliable treatment. The procedure was gradually developed to optimise the massive capital investment of the Flemish Government to comply with the effluent standards set in the EU Directive 271/91. The methodology makes an extended use of dynamic modelling. To facilitate the identification and estimation of the uncertainty, a probabilistic Monte Carlo engine is coupled to the dynamic models. In doing so this approach provides a way of explicitly incorporating uncertainty and variability in the process analysis. The procedure is examined in an upgrade of a conventional WWTP towards stricter effluent norms on nutrients. The results suggest that the procedure can support the decision-making process under uncertainty conditions and enhance the likelihood of meeting effluent standards not entailing above-normal capital investments. The analysis led to reducing the capital investment by 43%, producing savings of more than 1.2 million Euros.

Full-scale application of the IAWQ ASM No. 2d model

In the framework of the EU-funded TTP-UPM project (Technology Transfer Project--Urban Pollution Management) the waste water treatment plant (WWTP) of Tielt was modelled with the recently issued IAWQ ASM No. 2d model. Up to 41 % of the total COD load is originating from a textile industry. A measurement campaign was conducted during a period with industrial discharge and a period with only domestic sewage. The stop of the industrial discharge resulted in a highly dynamic response of the system. Based on an expert-approach the calibration was obtained changing only four parameters (anaerobic hydrolysis reduction factor etafe, reduction factor for denitrification etaNO3, the decay rate of autotrophs bAUT and the decay rate of the bio-P organism building blocks bPAO, bPHA, bPP). Influent fractionation remains a critical step within the model calibration. A proven procedure to characterise the influent determinants by standard physical chemical analysis failed to assess the influent COD fractions when the textile waste water is discharged to the WWTP. Selected bench-scale experiments, instead, succeeded in providing the adequate influent characterisation accuracy. For characterising the readily biodegradable COD fraction respirometry is to be preferred.