Treatment of septage in constructed wetlands in tropical climate: lessons learnt from seven years of operation.

In tropical regions, where most of the developing countries are located, septic tanks and other onsite sanitation systems are the predominant form of storage and pre-treatment of excreta and wastewater, generating septage and other types of sludges. The septage is disposed of untreated, mainly due to lack of affordable treatment options. This study presents lessons that have been learned from the operation of pilotscale constructed wetlands (CWs) for septage treatment since 1997. The experiments have been conducted by using three CW units planted with narrow-leave cattails (Typha augustifolia) and operating in a vertical-flow mode. Based on the experimental results, it can be suggested that the optimum solids loading rate be 250 kg TS/m2 yr and 6-day percolate impoundment. At these operational conditions, the removal efficiencies of CW units treating septage at the range of 80-96% for COD, TS and TKN were achieved. The biosolid accumulated on the CW units to a depth of 80 cm has never been removed during 7 years of operation, but bed permeability remained unimpaired. The biosolid contains viable helminth eggs below critical limit of sludge quality standards for agricultural use. Subject to local conditions, the suggested operational criteria should be reassessed at the full-scale implementation.     

Integrated natural treatment systems for developing communities: low-tech N-removal through the fluctuating microbial pathways.

Integration of natural treatment systems (NTS) (WSP, wetlands etc.) with each other as well as with advanced unit processes (biofiltration) offers a second lease of life to NTS. Long-term full and pilot cale experience in South Africa and Thailand have shown that contrary to a common view, a low tech N-removal from municipal and light industrial wastewater is a reality for a developing community The high treatment efficiency is ascribed to interplay of N-related processes complementing each other. The present FISH-based (Fluorescence In Situ Hybridization) approach to microbial community structure is a pioneering effort in the field of NTS. It establishes interrelationships between major N-removing groups (aerobic and anaerobic ammonia oxidizers (ANAMMOX), denitrifiers) within integrated systems and links them to the high treatment performance. Seasonally fluctuating presence of the ANAMMOX bacteria (0-2.5% of total bacterial numbers) in the NTS (free surface flow wetland) is reported for the first time. Their numbers correlate with metabolically dependent ammonia-oxidizers (2.0-3.0%) but not with stable overall Planctomycetes population (4.5-5.1%). As a result of the flexible microbial structure the robust low cost removal down to TN < 10 mg/L is routinely feasible at the loading rates ranging from 0.005 to 0.08 TN kg/m3/day.     

Removal of humic substances from natural waters by reverse osmosis

An investigation of the potential use of reverse osmosis for the removal of humic substances in order to remove colour and haloform precursors in small waterworks has been carried out, using three different laboratory scale reverse osmosis units and several different membranes.Membrane pore size was found to be the most important factor that influenced both the permeate quality and the product water flux. Pressure was found to have no significant influence on permeate quality, but was linearly related to product water flux. The concentration of humic substances in the influent was not found to affect product water flux but the transport of humics across the membrane was found to be dependent upon influent concentration. For the selected membranes, the removal of humic substances amounted to 80–100% in terms of colour removal, and 50–99% in terms of permanganate value reduction. The most suitable membranes for the different available units were found to be Osmonics SEPA 89 (permeate flux 251 m⁻² h⁻¹ at 15 bars), DDS 865 (permeate flux, 1201 m⁻²h⁻¹ at 40 bars) and PCI T2A (permeate flux 901 m⁻²h⁻¹ at 20 bars). At suspended solids concentrations higher than 100 mg 1⁻¹ of bentonite, product water flux was significantly reduced.     

Influence of sulfur-crosslinking in vulcanized rubber chips on mercury(II) removal from contaminated water

The adsorption of Hg(II) by natural rubber chips was investigated. First, the effect of chip size (5 mmx5 mm and 10 mmx10 mm) on the adsorption kinetics was studied. The pseudo-second-order modeling was found to explain the kinetics well. The smaller chips had higher adsorption rate so they were used for the rest of the research. Next the effects of sulfur, zinc oxide and carbon black on the adsorption capacity of Hg(II) at equilibrium conditions were investigated. The effect of sulfur was studied through different standard vulcanizing systems. The amount of zinc oxide was varied to be 3, 4 and 5 part per hundred parts of rubber (phr) while the carbon black (N-330) loading was varied to be 0, 30 and 50 phr, respectively. It was found that adsorption capacity increased with the degree of crosslink density, generated by sulfur reacting with rubber molecules. In addition, the adsorption capacities of various amounts of zinc oxide corresponded with their crosslink densities while the addition of carbon black seemed to obstruct Hg(II) adsorption.     

Degradation and detoxification of formaline wastewater by advanced oxidation processes

In this study, advanced oxidation processes (AOPs) utilizing the combinations of UV/H(2)O(2), Fenton, photo-Fenton and the combination of Fenton/photo-Fenton reactions were investigated in lab-scale experiments for the degradation of formaline wastewater. The studied toxic chemicals were formaldehyde and methanol in mixture solution, so-called formalin, which is the embalming agent in mortuaries. The experimental results showed that the photo-Fenton process was the most effective treatment process among the studied AOPs. Pseudo-first-order degradation rate constants of formaldehyde and methanol were obtained from batch experimental data. In the combination of Fenton/photo-Fenton reactions, the results show that applying UV light at an early stage of the reaction might not be necessary for a speedy oxidation reaction of the Fenton process. With Fenton and photo-Fenton processes, mineralization of formaline wastewater can be achieved, as no residual TOC is detected in the effluent after the reaction period. It is suggested that Fenton and photo-Fenton processes are viable techniques for the formaline wastewater treatment as they were able to provide high degradation of formaldehyde and methanol with relatively low toxicity of the by-products in the effluent.