Removal kinetic of Escherichia coli and enterococci in a laboratory pilot scale wastewater maturation pond.

During the last 15 years several authors studied the disinfection in waste stabilisation pond (WSP) and several empirical models were developed. There are huge differences between the models describing this process and there is really a need to improve the design of ponds for better disinfection. This paper addresses the Escherichia coli and enterococci disinfection in a laboratory pilot scale maturation pond (1.5 l) with light intensity (0, 12 and 25 W/m(2)) under controlled pH, temperature and dissolved oxygen (DO) conditions. The aim of this study is to improve modelling for a better design of disinfection in maturation ponds (MP) and to identify the key parameters influencing the process. It was found that kinetic coefficients K values for E. coli and enterococci are closely dependent on physicochemical parameters. K values increase with increasing pH, I, T and DO. E. coli disinfection depends closely on the pH and the DO and increases strongly when the pH is above 8.5. The enterococci disinfection depends essentially on DO. Two equations are suggested to calculate the kinetic coefficient K related to the environmental average state variables.

A model for anaerobic ponds combining settling and biological processes.

This work presents an approach to an anaerobic pond model by combining the stoichiometry of the hydrolysis and acidogenic processes of the main constituents of wastewater, i.e. carbohydrates, proteins, and lipids, grouped as a 'combined substrate' with a previously published settling model (see 'Suspended solids settling and half removal time in stabilization ponds (Tunisia)' by Effebi et al. (2011)). This approach includes biomass production. Coupling the kinetics and stoichiometry of the previous processes with the usual methanogenic model, we developed an anaerobic pond model. This paper gives the stoichiometry of the different chemical reactions that occur during the degradation of a conventional influent (corresponding to what we define as a 'combined substrate') of domestic wastewater and the model's first results.

Removal improvement of bacteria (Escherichia coli and enterococci) in maturation ponds using baffles.

Korba wastewater treatment plant is a conventional activated sludge followed by three maturation ponds (MP1, MP2, MP3) in series acting as a tertiary treatment. The first study of wastewater treatment plants showed that the effluent concentration of Escherichia coli and enterococci at the outlet of the (MP3) varies between 10(3) and 10(4)CFU/100 ml. After the hydrodynamic study conducted by Rhodamine WT which showed short-circuiting in the MP1, two baffles were introduced in the first maturation pond (MP1) to improve the hydrodynamic and the sanitary performances. The second hydraulic study showed that the dispersion number 'd' was reduced from 1.45 to 0.43 by this engineering intervention and the Peclet number was raised from 0.69 to 2.32. The hydraulic retention time was increased by 14 h. Because of well-designed baffles, the removal efficiency of E. coli and enterococci was raised between 0.2 and 0.7 log units for the first maturation pond.

Comparative analysis of existing disinfection models.

For a long time Marais's model has been the main tool for disinfection prediction in waste stabilization ponds (WSPs), although various authors have developed other disinfection models. Some ten other empirical models have been listed over the past fifteen years. Unfortunately, their predictions of disinfection in a given pond are very different. The existing models are too empirical to give reliable predictions: often their explanatory variables were chosen arbitrarily. In this work, we try to demonstrate that if influent variables have daily variations, the use of their average values in simulations may overestimate the disinfection effect. New methods are thus needed to provide better fittings of the models. Better knowledge of the mechanisms involved is needed to improve disinfection models.

Modelisation of the contribution of sediments in the treatment process case of aerated lagoons.

In aerated lagoons and even more in stabilization ponds the specific power (W/m3) is not high enough to maintain all the suspended solids in suspension. Some part of the suspended solids (including biomass) settles directly into the reactor and not in the final settling pond. The gradual accumulation of those sediments on the pond bottom affects performance by reducing the pond volume and shortening the Hydraulic Residence Time. However, the role played by these deposits is not restricted to such a physical effect. Far from being inert sediments they are also an important oxygen sink that must be taken into account when designing aerator power and oxygen supply, for example. On the other hand, under aerobic conditions, the upper layer of sediments may contribute to the treatment as a biofilm compartment in the reactor. In aerated lagoon systems another process contributes to the interaction of deposits and the liquid phase: the operating (often sequencing) of aerators may induce a drastic resuspension of deposits. In a 3,000 m3 aerated lagoon we evaluated that 3 tons of deposits were resuspended when aerators were started. Due to those processes we consider that a mathematical model of an aerated lagoon or of a stabilization pond has to take into account the contribution (positive and negative aspects) of deposits in the process. In this paper we propose a model for sediments including production but also biological processes. Simulations of the aerated lagoon with or without the "sediment compartment" demonstrate the effect and the importance of this compartment on the process. Of course a similar approach could be used for facultative or even maturation ponds. The next step would be to include anaerobic activities in the bottom layer.

Contribution of floating macrophytes (Lemna sp.) to pond modelization.

The objective of the present study was to develop a methodology for the quantification of the growth rate of Lemnaceae biomass by digital image analysis. The effect of biomass surface coverage on the oxygen transfer coefficient (Kla) was also quantified. Contribution of Lemnaceae to oxygen balance was evaluated by closed respirometry. Monod-like equations could be derived from growth rate coefficients in various experimental conditions. This opens the way to a deterministic model of Lemnaceae ponds where uptake of nitrogen and phosphorus (even heavy metals) can be calculated.

Nitrogen removal during leachate treatment: comparison of simple and sophisticated systems.

Membrane bioreactors (MBR) have become common in treating municipal wastewaters. Applied to leachates treatment MBR were also successful with pilot scale experiments and full-scale facilities as well. We succeeded previously in designing an efficient nitrification-denitrification process with an ethylene glycol byproduct as carbon source for denitrification. Moreover, an unexpectedly high inert COD removal efficiency was also observed in the full-scale MBR facility thereby making it possible to increase the operating time of the final GAC (Granulated Activated Carbon) adsorber. Since MBR are very sophisticated systems. Simpler and "lower" cost systems can also be considered. For example it is possible to nitrify leachates from sanitary landfill using a simple infiltration-percolation technique with a low energy cost. To validate previously published laboratory experiments, a semi industrial-scale pilot installation was installed at the Montzen landfill site (Belgium). The process is based on infiltration-percolation through a granular bed. This well known process was modified to increase the load, notably by changing the support medium, adding an electric fan that is run intermittently and maintaining temperatures greater than 15 degrees C. The new material is a type of granular calcium carbonate with a large specific surface area. These technical improvements enabled the system to nitrify up to 0.4 kg NH4+-N/m3 of reactor bed per day at a hydraulic load of 0.35 m.d(-1), with an ammonia removal rate in the range of 80 to 95%. Despite the high ammonia nitrogen inlet concentrations, this system exhibits remarkable nitrification efficiency. Moreover, these performances are achieved in a batch mode system without recirculation or dilution processes. If complete nitrification is needed, it can be obtained in a second in series of bioreactors. The system can be classified as a low cost process. An international patent is pending. Possible performances of those systems were compared with the usual methods for leachates treatment.

Dynamic mathematical model of high rate algal ponds (HRAP).

This article presents a mathematical model to describe High-Rate Algal Ponds (HRAPs). The hydrodynamic behavior of the reactor is described as completely mixed tanks in series with recirculation. The hydrodynamic pattern is combined with a subset of River Water Quality Model 1 (RWQM1), including the main processes in liquid phase. Our aim is to develop models for WSPs and aerated lagoons, too, but we focused on HRAPs first for several reasons: Sediments are usually less abundant in HRAP and can be neglected, Stratification is not observed and state variables are constant in a reactor cross section, Due to the system's geometry, the reactor is quite similar to a plugflow type reactor with recirculation, with a simple advection term. The model is based on mass balances and includes the following processes: *Phytoplankton growth with NO3-, NO2- and death, *Aerobic growth of heterotrophs with NO3-, NH4+ and respiration, *Anoxic growth of heterotrophs with NO3-, NO2- and anoxic respiration, *Growth of nitrifiers (two stages) and respiration. The differences with regard to RWQM1 are that we included a limiting term associated with inorganic carbon on the growth rate of algae and nitrifiers, gas transfers are taken into account by the familiar Adeney equation, and a subroutine calculates light intensity at the water surface. This article presents our first simulations.

Comparison of oxygen and carbon dioxide balances in HRAP (high-rate algal ponds).

We previously suggested a method to characterize the oxygen balance in High-Rate Algal Ponds (HRAPs). The method was based on a hydrodynamic study of the reactor combined with a tracer gas method to measure the oxygen transfer coefficient. From such a method diurnal variations of photosynthesis and respiration can be quantified and the net oxygen production rate determined. In this paper we propose a similar approach to obtain carbon dioxide balances in HRAPs. Then oxygen and carbon dioxide balances can be compared.

A system for on-site performance evaluation of small wastewater treatment plants according to the European procedure.

The new European standard project for performance evaluation of on-site systems (prEN 12566-3) provides a test procedure at user sites for individual on-site wastewater treatment. For each system, two plants of the same size have to be tested, one operating at 50% and the second one with a load greater than 75% of the nominal capacity. The test duration is one year and several stress tests are included in the program (holidays, bath water discharge and power breakdown). Flow based composite 24 h samples, on the influent and the effluent have to be used, with 30 day intervals for influent and 15 days for effluent. On each sample, the following measurements have to be done: BOD5 or COD, suspended solids, temperature, power consumption, daily flow. It appears that very few systems have been reported in the literature to follow up such facilities, especially describing how to sample at the inlet of on-site individual equipment. In order to obtain representative samples at the inlet and at the outlet of those on-site treatment systems, we have designed an original mobile sampler system. The whole system ensures the flow measurement and flow based sampling as well. In this paper, we present the different parts of the sampling system (pump, flowmeters, ...), its validation and the results obtained at 5 user sites during the first 9 month period. Preliminary results are very interesting because they clearly demonstrate the need for an efficient on-site control of those user sites and for better legislation in this domain.