Performance indicators for wastewater treatment plants.

The Swedish Water & Wastewater Association has operated a web-based system, VASS, for the collection and compilation of key data from the Swedish water utilities since 2003. The VASS system will now be expanded to include data on operation of individual wastewater treatment plants (WWTP). The objective is to provide performance indicators (PIs) for performance and economy and the use of resources such as energy, chemicals and manpower. A set of PIs has been developed that also includes explanatory factors to compensate for differences in the condition of operation between plants. This paper discusses the data required for the calculation of PI but also for explanatory factors, quality checks and for plant operation context. The discussion is based on the experiences from a test round with the participation of 24 WWTP.

Centralised or decentralised sanitation in Swedish summerhouse areas in transition to permanent living?

The standard of wastewater management is high in Sweden. Around 90% of the population is connected to central wastewater treatment plants with high requirements of nutrients removal; however, still the problem with algae blooms in the Baltic Sea exists. The aim of the VeVa project was to develop a simple and user-friendly Excel-based model to support environmental decisions of how to select wastewater systems for housing areas where no central sewer system exists. The VeVa model deals with two types of environmental issues: substance flow analysis and energy analysis. Six system structures were studied for the transition area Lillängsdal in Värmdö municipality sorted in three categories: 1) on-site systems for single households; 2) local collective systems; 3) connection to central systems. All studied system structures, except for a Sand filter system, fulfilled the goals of reducing phosphorus and BOD7 according to Swedish guidelines for on-site systems in sensitive areas. All studied systems, except for the Sand filter system, have the potential to fulfil the Swedish National Environmental goal to recycle 60% phosphorus to productive land. The systems with central wastewater treatment plant and local wastewater treatment are the most energy efficient alternatives that also fulfil the requirements of discharges and environmental goals regarding phosphorus recycling.

Wastewater treatment with anaerobic membrane bioreactor and reverse osmosis.

Domestic wastewater from a new city district in Stockholm has been treated by an anaerobic membrane bioreactor (AMBR) followed by reverse osmosis (RO). The main objectives were to study the gas production, the reduction of organic matter and nutrient recovery. The AMBR was operated at 22 degrees C (equal to the average temperature in the influent) and a hydraulic retention time of 0.6 d. The results show that the reduction of organic matter, nitrogen and phosphorus over the AMBR was approximately 92, 9 and 9%, respectively. A stable gas production was registered throughout the evaluation period. The overall removal efficiency, i.e. including the RO, was >99% for TOC, >91% for Kj-N and about 99% for P. Adding a reverse osmosis (RO) unit to the AMBR makes it possible to produce a concentrated, nutrient rich product well suited for agricultural use. The quality of the concentrate is, in terms of nutrient concentration and heavy metal content, similar to source separated human urine, i.e. nitrogen content about 3 g N/L and <2 mg Cd/kg P. However, addition of acid is required to prevent precipitation/fouling of the RO. The total electricity use for operation for the system, including the RO-unit, is estimated to be 3-6 kWh/m3.

Concentration of nutrients from urine and reject water from anaerobically digested sludge.

Experiments with concentration of nutrients from source separated urine and reject water from digestion of sludge in sewage treatment plants (STP) have been performed in laboratory and pilot scale. The methods tested were membrane filtration with reverse osmosis (RO), evaporation, and precipitation of phosphorus and distillation of ammonia. In membrane filtration, pre-filtration with particle separation at 5-10 microm was enough to avoid clogging of the membranes. Separation of phosphorus (P), potassium (K) and sulphur (S) was almost 100%, while separation of nitrogen (N) was dependent on pH. The capacity of flux increased with temperature and pressure. In evaporation, all P, K and S were still in the concentrate, while pH had to be decreased to 4.5 to avoid significant loss of N. In precipitation and distillation, about 90% of P could be recovered from urine as magnesium ammonium phosphate (MAP) just by adding MgO. For the reject water pH was first increased by aeration to remove CO2. Ammonium can be distilled from the water phase after precipitation of MAP, without further increase of pH. At least 80-90% of N can be distilled in 5-10% of the total volume. The article also discusses the quality of different products, cost of separation, and energy and chemical demand.

Exergy analysis of nutrient recovery processes.

In an exergy analysis, the actual consumption of resources in physical and chemical processes is calculated. Energy and chemical elements are not consumed in the processes--they are only transformed into other forms with lower quality. The principals of exergy analysis are illustrated by comparing different wastewater treatment systems for nutrient recovery. One system represents an end-of-pipe structure, whereas other systems include source separation of grey water, black water, and urine. The exergy flows analysed in this paper are those related to management and treatment of organic matter and nutrients. The study shows that the total exergy consumption is lowest for the system with source separation of urine and faeces and greatest for the conventional wastewater treatment system complemented by processes for nutrient recovery.

Evaluation of small wastewater treatment systems.

Different technologies for the treatment of wastewater from single houses have been evaluated by performing measurements at 14 treatment plants installed at different one-family houses located 35 km south-west of Stockholm. The technologies tested were selected in order to deliver high removal efficiencies with respect to organic matter, nitrogen and phosphorus. They also had to fulfil requirements concerning robustness, nutrient recycling, use of natural resources, economy, user-friendliness and hygiene. The systems can be divided into package plants, urine separating systems, blackwater separating systems and chemical precipitation as a supplement to sand filter beds. All of the systems were able to remove more than 90% of phosphorus and more than 90% of organic matter. Concerning package plants, it is necessary to have reliable dosing equipment and frequent checks to achieve long-term phosphorus removal. The source separation systems require well-informed and motivated users to achieve desirable removal efficiency. Sorting systems seem to be superior with respect to nitrogen removal, even though some of the package plants gave nitrogen removal above 50% during long periods of operation.

Systems analysis for environmental assessment of urban water and wastewater systems.

In this paper, two fundamentally different urban wastewater systems are assessed from an environmental impact perspective. One system represents a centralised, high-tech, end-of-pipe structure whereas the second system is primarily based on source-separation strategies. Using material flow analysis in combination with evaluation methods based on life-cycle assessment the systems are evaluated by means of simulation and the results are discussed. A set of priority indicators for environmental assessments of urban water systems is suggested and applied in the analysis. Energy issues are also commented upon. The main intent of the paper is to present the principles of this type of assessment rather than detailed numbers for all possible environmental effects and hazardous substances emitted to air, water and soil. It represents one of several building blocks for a future multi-criteria decision-support system to evaluate urban water management from a sustainability perspective.