Small Wastewater Treatment Research Articles

Optimization of Nitrogen Removal in Small Activated Sludge Plants

In France, many small sewage treatment plants are of the activated sludge/extended aeration type and, generally, only receive a part of their nominal organic load. They are mostly equipped with surface aerators whose Standard wire Aeration Efficiency is in the range 1.3-1.9 kg 02 kwh−1. Consequently the sludge age and the supply of oxygen are sufficient to obtain a large elimination of nitrogen from domestic waste water even when the sludge temperature is low. However the concern is with the optimizing of the nitrogen treatment by the nitrification of ammonia in the aeration basin whilst avoiding any parasitic denitrification in the clarifier. Amongst other investigations, 4 small domestic wastewater treatment plants were studied over several months. A number of adjustment modifications were imposed on them in order to optimize the elimination of nitrogen : The final concentrations to be reached were : N-NH4− lower than 5 mg.l−1 and N-NO3− lower than 3 mg.l−1. The daily operating time of the aerators depends on the received load, the sludge concentration and the oxygenation capacity. When the works are under-loaded, the non-operational periods of the aerators should be as long as 1 1/2 and 2 hours in order to achieve the nitrates reduction. When the load is higher, the time required to attain anoxia after the shutdown of the aerators is shorter and the length of the non-operational periods can be reduced. These field experiments have allowed an evaluation of the tolerances around an optimum adjustment of the aeration operation : a reduction of 5 to 10 % of the daily aeration time may bring about an increase in the residual concentration of Kjeldahl nitrogen which reaches 10 rag.1−1. Conversely, sludge losses may take place in the clarifiers if the daily aeration time is increased by 5 to 10 % in the case of completely mixed basins equipped with slow vertical shaft aerators, and more than 15 % in the case of oxidation ditches (better denitrification probably due to the continuation of the current after stopping the rotors). The sludge concentration should be kept within fairly tight limits (± 0.5 g.1−1). It is however essential to rectify the aeration adjustments during the seasons : a rise in the sludge temperature of 10°C generates an increase in the total oxygen demand by 3 to 5 %. The particular case of plants with anoxic zone ahead is finally discussed.

Urban Drainage Strategies for Small Communities in Developing Countries

In this paper, the interactions of small waste water treatment plants within an urban drainage system are pointed out. Especially in developing countries, the characteristics of catchment areas differ very much from the ones in industrialized countries. Consequently, the plants have to be adapted to the local conditions of drainage, climate and location. Finally, an urban drainage strategy will give an idea how to sanitize drainage systems.

Biofiltration as a Compact Technique for Small Waste Water Treatment Plants

Biological aerated filters combine bacterial degradation of pollution by fixed biomass with physical filtration in a single reactor. Removal rates become independent of clarification and sludge setueability limits, and concentration of biomass is increased. Nitrifiers attach to the media, allowing nitrogen removal without sludge age constraints. Several fullsize plants with the BIOCARBONE process for industrial and municipal wastewater treatment have established the compacity, ease of operation and high removal rates achievable with this advanced treatment system. A new biofilter design offering simplified operation and increased performance is presented, which allows implementation of biofiltration for small wastewater treatment plants. Design data for carbon and nutrient removal were collected during extensive pilot tests. Hydraulic conditions and pollution loadings were varied in order to optimize the biological and operational parameters of the filter. The combination of an anaerobic and an aerobic zone eliminates the need for primary sedimentation. Pollution removal rates up to 20 kg COD/m3 d could be achieved, and a widely fluctuating load of up to twice that average loading can be treated without major effluent deterioration. If lower carbon loadings are used, nitrification is achieved in the upper aerated zone. By recirculating the effluent into the non-aerated lower zone, carbon and ammonia oxidation as well as denitrification and suspended solids retention could be achieved with an overall hydraulic retention time of four hours in one reactor.

The Experience and the Results of Operating Small Wastewater Treatment Plants

About 15 years of experience gained from operating 7 small waste water treatment plants between 150-500 p.e. and 6 plants of 2000-3000 p.e. capacity are described. All of the plants involved are low-loaded activated sludge plants with water jet aeration generated by screw or propeller pumps which both proved to be efficient in aeration while secure and simple in operation. In comparison with other types of small treatment plants in our country (rotating discs and bubble aeration activated sludge plants), they proved to be reliable and simple in operation and produce an effluent of good quality. The energy consumption and the necessary flow velocites in aeration tanks are computed and measured.

The Operation and Maintenance of Small Scale Wastewater Treatment Plants

Three wastewater treatment processes (oxidation ditch, extended aeration (plug flow type) and activated sludge batch reactor) which are considered to be suitable for small scale wastewater treatment plants were tested at a pilot plant, or using existing facilities to examine their methods of operation and maintenance. The objectives of this study were to determine production of excess sludge, a method of setting aeration intensity, and a method of controlling returned sludge.