Current Trends in Biological Wastewater Treatment

DOI: 10.2478/alife-2018-0055 CURRENT TRENDS IN BIOLOGICAL WASTEWATER TREATMENT Dragos DRACEA, Augustina TRONAC, Sebastian MUSTATA University of Agronomic Sciences and Veterinary Medicine of Bucharest, 59 Marasti Blvd, District 1, Bucharest, Romania Corresponding author email: [email protected] Abstract Waste water treatment plants are complex systems consisting of construction, hydro-mechanical, electrical, monitoring and automation equipment. Monitoring activities emphasize that the processes are dynamic; wastewater quality at the entering point of treatment plant varies in a wide range. Treatment technologies adopted must reduce major pollutants; that involves nitrification-denitrification processes and biological and chemical reduction of phosphorus through mechanical-chemical-biological treatment pathways. Increasing the activated sludge concentration in a wastewater treatment plant is an effective method by altering the process dynamics and by reducing the produced sludge volume. There are proposed and discussed in terms of technical and cost efficiency different technological wastewater treatment schemes. In Romania, wastewater treatment plants and sewage systems operating involve processes based on the new systems overrated, there is mandatory to diminish quantities in water supply systems and to exclude improperly working of wastewater pre-treatment stations. Those operations impose technological measures ensuring efficient functioning regardless the service conditions. Keywords: biological treatment, technical efficiency, nitrification-denitrification processes, activated sludge. possibilities. Technological development and environmental conservation concept has led to narrow pollutant concentration limits for water discharged into the emissary. In Romania, treatment technologies adopted must ensure the reduction of major pollutants (nitrogen and phosphorus nutrients) in new wastewater treatment plants or in rehabilitated ones. The technologies adopted involve nitrification-denitrification processes and biological and chemical reduction of phosphorus through mechanicalchemical-biological treatment pathways. Sludge production increments from new and rehabilitated wastewater treatment plants impose many ways to dispose it (Feodorov V., 2017). Biological treatment systems adopted have highlighted disadvantages and advantages; so, no technology stands out from technical, economical or investitional point of view. Technological treatment alternatives commonly used in Romania have been (www.asio.ro, 2017; www.adiss.ro, 2015):  SBR sequential systems or similar;  systems with predenitrificare-nitrification and related recirculation; INTRODUCTION Wastewater treatment plants are complex systems consisting of construction, hydro-mechanical, electrical, monitoring and automation equipment. Simple systems involve reduced investment and operating costs, but they develop sensitive processes and they could be destabilized when there are variations in operating conditions. Complex installations properly monitored emphasize that the processes are dynamic, wastewater quality at the entering point of treatment plant varies in a wide range. In Romania, wastewater treatment plants and sewage systems operating involve specific processes based on the new systems overrated, necessity of water quantities diminuation for water supply systems and also on improperly working of wastewater pre-treatment stations. RESULTS AND DISCUSSIONS Treatment systems Due to the high water use, most laws imposed in the field, provide a maximum degree of purification, at the edge of technological 373 reactor, a good transfer of oxygen in the water. Operating difficulties are due to the impossibility of proper aeration system tuning, detachment of biological mass fixed and driving it to the exhaust system. Pollutant concentration variation at the treatment plant entering point may involve oxygen deficit and the need of reactor over aeration. Technological pathways with sequential processes involves recirculation reduction for denitrification; that leads to important variantion of ammonia nitrogen and nitrates/ nitrites concentration within treated water. Wastewater treatment plants with successive phases have difficulties at denitrification phases, in particular when operating with reduced inputs and require monitoring systems for each compartment. In these circumstances, combining these elements into a wastewater treatment plant and a suitable supply system can provide an optimal operation. It could be identified a number of technological schemes with predenitrification or partial nitrification followed by sequential basins. Predenitrification technological variants (Figure 1 and Figure 3) ensure oxidized nitrogen removal from external recirculation flow which may be increased in order to enhance the efficiency. Limiting the supply flow Q1 ensures technological time growth, activated sludge concentration increasing c1> c2> c3 in the first biological basins, treatment efficiency enhancing and reduces the burden on the secondary sedimentation tank. Feed flow Q2 balances the carbon into the process phases, especially for the denitrifycation phase. Mostly, the sequential operation basin will work in order to realise denitrification phase, nitrification stage having only a corrective role. Correction flow Q3 relieves the burden on installations, eliminating partially the pollutants near to the evacuation accepted limit. Technological scheme with partial nitrification (Figure 2) or combined with predenitrification on the recirculation flow (Figure 3) allow the use of a biomass fixing mounting bracket fixed or mobile; biological concentration is high ensuring maximum process yields.  systems with nitrification-denitrification;  systems having oxidation ditch;  sequentially systems with secondary sedimentation tanks and recirculation;  systems with nitrification-denitrification successive phases. Systems without appropriate recirculation proved to be sensitive to significant pollutant load variations at the entrance point of wastewater treatment plant (flow rates, concentrations); recirculation systems have involved additional costs of energy and denitrification phase is possible to be applied to low input loads. In these conditions, there are required analysis and adoption of technical solutions that provide increased efficiency and reliability in the context of cost reducing: - increasing activated sludge concentration in bioreactors and adoption of performant sedimentation tanks or exhaust systems with vacuum filters (www.cv-water.ro, 2016); - increasing activated sludge concentration by fixing of biological mass in bio filters (fixed or mobile) (www.asio.ro, 2017; www.adiss.ro, 2015, www.cv-water.ro, 2016); - wastewater treatment plant combining sequential phases with successive phases (www.adiss.ro, 2015); - balancing processes through controlled supplying of biological compartments (Ionescu I.A., Bumbac C., Cornea P., 2015); - active sludge quantity control (Racoviteanu R., Ionescu Gh. C., Ianculescu O., 2011] and quality control considering persistent organic compounds (Preda M., Tanase V., Vrinceanu N.O., 2017), also specific biological products use. Biological processes optimization in wastewater treatment plants Increasing the activated sludge concentration in a wastewater treatment plant is an effective method by altering the process dynamics and by reducing the produced sludge volume. Operation of such wastewater treatment plant involves oversizing of sludge management systems (rinse, recirculation) to ensure the exploitation under high flow rates (cv-water.ro, 2016). Using systems for biological biomass fixing provides by maintaining sludge into biological 374 Q1 Q2 Q3 N N/DN DN DS c3 c2 c1 External recirculation Figure 1. Technological scheme with denitrification on the recirculation flow and sequential biological basin: DN-denitrification basin, N- nitrification, N/DN- nitrification-denitrification sequential basin, Q1, Q2, Q3- split supply, c1>c2>c3- activated sludge concentration Q1 Q2 Q3 Nf c1 N N/DN c3 DS c2 External recirculation Figure 2. Technological scheme with partial nitrification and sequential biological basin:Nf-partial nitrification basin with or without biological fixing environment, N- nitrification, N/DN- nitrification-denitrification sequential basin, Q1, Q2, Q3- split supply, c1>c2>c3- activated sludge concentration the optimal technological solution including an airlift pump. By the operation of the proposed schemes, the gravity centre of biological processes is shifted to inlet compartments, the final one becoming a technological reserve compartment. For providing an optimum process there are needed monitoring activities of the oxygen in nitrification pond and of the ammonia nitrogennitrate values in denitrification basin. Maintaining high concentrations of biological mass away from secondary sedimentation tank is achieved by fractional supply through Q2 and Q3 components. The ending nitrification phase, before decantation and water evacuation, helps denitrifycation block, ensures required correction for unoxidized nitrogen from Q2 and Q3 flows and eliminates any floating processes from the sedimentation tank. The system does not exclude recycling into final nitrification and interdenitrification phase, 375 Q1 Nf Q2 Q3 c1 DN N N/DN c3 DS c2 External recirculation Figure 3. Technological scheme with predenitrification on recirculation flow, partial nitrification and sequential biological basin: Nf- partial nitrification basin with or without biological fixing environment, N- nitrification, N/DNnitrification-denitrification sequential basin, Q1, Q2, Q3- split supply, c1>c2>c3- activated sludge concentration CONCLUSIONS REFERENCES Complex wastewater treatment schemes implicate investment and operating high costs. Operation imposes technological measures ensuring efficient functioning regardless the service conditions. 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