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BIA-Brukerstyrt innovasjonsarena

Sustainable treatment of wastewater by Salsnes Filter fine mesh sieves and biological processes

Awarded: NOK 6.5 mill.

Project Number:

211055

Project Period:

2011 - 2015

Funding received from:

Partner countries:

Salsnes Filter and SEFAR have worked on development of new sieve cloths, new sieves and a new control system for removal of biofilm solids. At Nordre Follo WWTP Aquateam COWI and UiS worked on separation of biofilm solids in laboratory scale and bench-scale. The WWTP has seven reactors in series. The particle size distributions (PSD) were found to vary according to organic loading on the individual reactors, i.e. higher organic loading resulted in smaller particle volumes and the particle size peaked at 100 microns in diameter. About 90 % solids removal with 18 micron fine mesh sieve was achieved for reactor 5 and reactor 7 unflocculated effluents. With an 11 micron fine mesh sieve a final effluent concentration of 8 mg SS/L was achieved. Flocculation of the biofilm biomass changed the particle size characteristics of the reactor effluent and the hydraulic capacity of the sieve cloths. Cationic polymers performed the best. The mixing and flocculation program selected for pilot-scale flocculation was 20 s of rapid mixing at 300 rpm followed by 15 min of slow mixing at 50 rpm. Continuous measurement of floc sizes during flocculation resulted in best floc formation after 3 minutes. Image analysis of the flocs suggests stirrer design and flocculant have an influence on the shape and structure of the flocs. SS removal with polymer flocculation was very good using a 210 microns sieve cloth. Tests have continued with pilot-scale testing on a SF 500 prototype sieve, and technical scale testing on a SF 1000 sieve. Significant improvements have been made with respect to cleaning of the sieve cloth, reduction of energy consumption, and increased removal efficiencies. The control system has been improved. More than 95 % TSS removal has been achieved in continuous operation. UiS and Aquateam COWI have worked on finding optimum particle removal before biological nutrient removal processes. Biological wastewater treatment for nutrient removal can be improved by taking into consideration organic materials (COD) entering the biological system. About 70 % of influent COD is associated with suspended solids (SS). Therefore an efficient SS and COD removal upstream (without negative effects on N and P removal) minimizes energy consumption in the aerobic system and could also increase organic waste for energy recovery from anaerobic sludge digestion processes. Bench scale reactors with AS (activated sludge) and MBBR (Moving Bed Biofilm Reactor) processes, respectively, were used to investigate denitrification rates (DNR) with selective removal of particulate COD (pCOD) using Salsnes Filter fine mesh sieves. Reactors were fed with untreated or sieved/filtered municipal wastewater from two different plants in the Oslo area. As an example 33 micron filtration reduced the DN-rates by 5-12 % compared to unfiltered wastewater, depending on the use of the initial or secondary DN-rates, the type of test reactor and the source of the wastewater. In sum, the effects of pCOD removal on DN rates were not very substantial with the AS and MBBR batch tests. These tests have been followed by continuous operation of laboratory scale activated sludge SBR-systems. It was concluded that particles larger than approximately 30 microns did not improve the denitrification rates or the removal of nitrogen. Removal of particles larger than approximately 30 microns was actually very beneficial with respect to increasing the nitrification rates in the reactors and hence achieving the required nitrogen removal within smaller reactor volumes. Fine mesh sieves also increased the amount of primary sludge available for anaerobic digestion and biogas production, and significantly reduced the production of biological sludge. To verify the findings of the optimum sieve openings, four parallel pilot-scale trains for biological nitrogen removal were tested in continuous operation. Two trains were MBBR processes, one with 2 mm sieve pretreatment and one with 33 microns fine mesh sieve primary treatment. The other two trains were MBR (Membrane BioReactor) processes with 2 mm pretreatment and 33 microns primary treatment, respectively. The benefits of using a 33 microns sieve were confirmed. The MBBR train with 33 microns SF primary treatment had the same denitrification rates, 10-15 % higher nitrification rates and 36 % lower oxygen demand than the control train. Bench-scale tests have been performed at different temperatures, in order to document the temperature dependency of the denitrification process. Full-scale testing in the Netherlands has confirmed that the fine mesh sieves can be controlled to produce a ratio between total COD and total N in the influent to the biological stage that will be the optimum for the biological process. At the Aartselaar WWTP the fine mesh sieve removed 37 % SS, 25 % COD and 6.7 % total N, resulting in a carbon to nitrogen (C/N) ratio of 8.1 g COD/g N in the sieve effluent.

Salsnes Filter fine mesh sieves have been very successful for primary treatment of municipal wastewater. However, the market for primary treatment is very small and decreasing. To open up a significantly larger market it is necessary to qualify the Salsn es Filter fine mesh sieves for use in combination with biological processes. The primary objective is to develop fine mesh sieve systems for primary and secondary solids separation at wastewater treatment plants, with the intent to save resources by re ducing the cost and energy consumption for biological wastewater treatment and maximizing the energy recovery from the organic matter in the removed solids. Secondary objectives: a) Document the optimum particle removal in front of biological processes , for optimum biological treatment performance and recovery of resources from particles in the wastewater. b) Develop a fine mesh sieve system for achieving the optimum particle removal under secondary objective a) at the lowest possible use of resources. c) Develop a fine mesh sieve system for solids separation downstream of biofilm processes, providing good, economic and energy efficient solids removal in a small footprint. d) Large scale verification of the fine mesh sieve systems for primary and secon dary (biofilm) solids separation. The optimum particle removal from municipal wastewater in front of different biological processes is not known. It will require PhD level research to develop a sieve and control unit that can achieve the desirable parti cle removal. Separation of biofilm biomass will depend on the properties of the biomass, which will change with type of process, operating conditions and wastewater composition. Comprehensive testing will be required to gain the knowledge needed to devel op, design and operate fine mesh sieve systems for these purposes. Furthermore, independent R&D institution documentation from this project is crucial for the marketing and sales of these systems.

Funding scheme:

BIA-Brukerstyrt innovasjonsarena