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Three conferences on “Small Wastewater Treatment Plants” organized by the IAWQ Specialist Group demonstrate worldwide interest and activities in this matter and the need to exchange experience concerning planning, design, construction, operation, maintenance and control of small treatment plants. In near future, the number of small treatment works will increase tremendously and will be accompanied by a strong demand for information on appropriate procedures and technologies. Pollution problems caused by small wastewater flows are usually restricted to small areas, however, in view of the high per capita costs, treatment requirements and alternatives have to be studied carefully. In comparison to larger plants, more pronounced and different boundary conditions such as load fluctuations, operation and maintenance problems, per capita costs, and a large variety of feasible treatment and disposal systems ask for experienced engineers with a broad and sound knowledge in rural water quality management. The technical alternatives reaching from mechanical and simple biological low rate systems such as ponds, sand filters and reed beds to complex high rate suspended and fixed biomass reactors have to be evaluated regarding plant size, operation safety, reliability, demand for skilled personnel, investment and operation costs. In this respect, water engineers are increasingly challenged, not only to deal with a broad range of present and future treatment technologies, but also to integrate economical and social aspects into their evaluations.

Sand filters as an appropriate treatment facility for small wastewater flows were investigated to determine solute and gas transport under the highly dynamic conditions of intermittent flushing. The effect of biomass accumulation on solute breakthrough was assessed experimentally by tracer studies. With the help of a one-dimensional subsurface water and solute code, solute transport and air diffusion were evaluated mathematically. The code was used to extrapolate to different hydraulic conditions. A second multiphase transport code enabled to include the effects of water and air flow simultaneously. From the calculated results, which were confirmed by gas tracer experiments, oxygen fluxes due to advective and diffusive transport phenomena could be evaluated. The better insight into transport limitations in intermittent filters allows some conclusions for design and operation of these systems.

A new method is presented which allows emissions of traffic into the environment to be described as a function of road distance. The method distinguishes different types of emissions (runoff, spray and drift), which are determined by measurements and mass balances of a specified road section. The measurement of two-dimensional pollutant concentrations in the road shoulder is an important part of the method. In a case study performed at Burgdorf, Switzerland, the method was applied to the determination of the spatial distribution of heavy metal emissions. The results show that between 36 and 65% of the heavy metals Cd, Cr, Cu, Pb and Zn are present in runoff and spray and between 35 and 64% are dispersed diffusely in the environment (defined as drift). The runoff infiltrates into the vegetated road shoulder up to a distance of approx. 1 m from the road. The distribution of spray shows a maximum at 1 m and decreases steadily up to a distance of 5 m. This information can serve as a basis for the quantitative evaluation of road-runoff treatment scenarios. Although the results of the Burgdorf study are case-specific, several general guidelines for the reduction of traffic-related emissions can be derived from it.

The first full-scale contact filtration plant for advanced phosphorus removal in Switzerland was built 1979. In a twelve month investigation, the following design and operational aspects were considered: optimal chemical conditions for phosphorus removal, multimedia filter configuration, long-term performance of the total process combination (which included conventional treatment with simultaneous precipitation and contact filtration), and operating variables as filtration velocity, filter run time and backwash water use. Under optimal conditions, contact filtration provided consistently low phosphorus and suspended solids levels at reasonable cost.

Based on 20 months of pilot experiments, tertiary plastic media trickling filters have shown to be a feasible and cost-saving solution for the enlargement of existing treatment plants for nitrification. The influence of several process variables such as hydraulic load, media characteristics, NH4-load, concentration fluctuations, suspended solids, organic nutrients and biomass grazing on nitrification activity was evaluated. From performance analysis along the trickling filter depth, basic design informations were obtained. In addition to suspended solids removal, subsequent deep-bed filtration proved to be an excellent final treatment to further reduce and stabilize effluent ammonia and nitrite residuals.

A nitrification model for a tertiary trickling filter is developed based on stoichiometry, Fick's Law and Monod kinetics. The design of tertiary trickling filters for nitrification is discussed, with special emphasis on: residual ammonium concentration, recirculation, reactors in series, residual alkalinity, residual nitrite concentration and effects of temperature on reactor performance. Wherever possible the theoretical predictions are compared with experimental results.

Solubility equilibrium models and/or adsorption models enable the prediction of phosphorus residuals in an effluent of a wastewater treatment plant over a wide range of operating conditions. A generally valid model cannot be given because the model parameters depend on wastewater composition, e.g. on dissolved and particulate water constituents, above all on phosphorus speciation. Chemical precipitation is able to bind ortho-phosphates proportionally to added chemicals and polyphosphates to a great extent. The P-residuals consist mainly of organically bound phosphorus and some residual polyphosphate. In wastewater treatment plants with enhanced biological phosphorus removal (EBPR), an extensive use of precipitation chemicals could suppress the EBPR-ability of the sludge. In EBPR-plants a part of the phosphorus can also be eliminated by chemical precipitation without added chemicals. In experiments with inactivated sludge derived from a wastewater treatment pilot plant, a pH-sensitive and partly reversible precipitation of calcium phosphates was observed at pH values below 8.0. A dynamic model was formulated on the basis of these observations. It contains the following reactions: – Fully reversible precipitation of hydroxydicalcium phosphate (Ca 2 HPO 4 (OH) 2 , HDP) as an intermediate, with a solubility product of 10 −22.6 (kmole m −3 ) 5 (20°C). – Formation of hydroxyapatite (Ca 5 (PO 4 ) 3 OH, HAP) from HDP. HAP is regarded as a kinetically stable endproduct. If the solubility of HDP is exceeded, then 15.5 gpm −3 d −1 of inorganic calcium phosphate (HAP) is fixated irreversibly in the sludge.