A Novel Method to Eliminate Side Streams in The Conventional Activated Sludge System Through En-Mbbr Process
Abstract
The sidestreams (supernatant gravity thickener, underflow belt thickener, and centrate) are the water generated from the sludge treatment stage units. The sidestreams contain high concentrations of pollutants, especially nutrients, and are returned to the head of the plant. Returning the sidestreams to the plant's head causes several problems, including technical and economic ones.
This thesis aims to eliminate the sidestream and treat it independently by a novel reactor and integrate the treated water from it with the effluent of the plant with achieving technical, economic, and environmental feasibility. The study was conducted on a sewage treatment plant for the city of Karbala in Iraq, which operates with the conventional activated sludge system type A2/O process.
The methodology of the research was through two axes, the first is the design and manufacture of a new system that performs the task of treating all the sidestream lines, and this system is called extended nutrient-moving bed biofilm reactor (EN-MBBR). The pilot reactor was divided into three stages: the first is for the treatment of organic pollutants using heterotrophic bacteria, the second is for the treatment of nutrients (NH4+H2S) using autotrophic bacteria and phosphate (PO4-P) chemical removal by adding calcium hydroxide, and the third is lamella clarifier to remove suspended solids. As for the second axis, it was by designing a full-scale EN-MBBR system proposed by GPS-X simulation to carry out the task of treating all the sidestream lines and integrating them with the plant’s effluent.
The results showed that the discharge ratio of the sidestreams to the mainstream is 4%. The ratio of pollutants present in the sidestream to the mainstream is observed in BOD, COD, TSS, NH4, PO4, and H2S were 4.3%, 4.7%, 8.8%, 21%, 40%, and 11% respectively. The pilot system successfully started and reached a steady-state in 28 days. The pilot system processed 30 m3/d of the sidestreams, and the average 8-months effluent concentrations for COD, BOD, TSS, PO4, NH4, NO2, NO3, H2S were 55, 4, 11, 0.16, 0.2, 0.17, 100, and 0.11 mg/L, respectively. Adding 3 kg/day of calcium hydroxide contributed to improving the nitrification process and reducing phosphates from 40 mg/L to 0.16 mg/L, but it caused an initial shock to the system that lasted more than a week, and then it was stabilized. The effects of dissolved oxygen (DO), return activated sludge (RAS), and media portion on the pilot were modeled using GPS-X. Modeling results showed that DO concentrations affected the nitrification process but stabilized at a concentration of 3 mg/L. When operating the EN-MBBR as an extended nutrient-integrated fixed activated sludge (EN-IFAS) system, the RAS has positively contributed to reducing sludge in this system, whereas the sludge proportion is reduced by 60%. The media portion had a significant effect on the removal of nutrients, as it gave the best results when the rate of reactor filling with carriers was between 40 to 50%.
The statistical results of R value greater than 0.8 and RMSE values near zero proved the calibrated model's validation. The novel system successfully removed 98, 93, 100, 85, 98, 100, and 98% of BOD, COD, NH4, NO3, TSS, H2S, and PO4-P from sidestreams, respectively. Furthermore, the simulation results showed that eliminating sidestreams has reduced volumes of full-scale A2/O facilities, controlled hydraulic and pollutants shocks, and minimized cost and energy. The novel process proved successful in treating combined sidestreams and eliminating their impacts on the A/O2 system.
The behavior and fate of pollutants were studied in the A2/O system by the GPS-X model, where high concentrations of orthophosphate and hydrogen sulfide were observed in the anaerobic zone. While all other pollutants begin to decay, phosphates and hydrogen sulfide gas also begin to decrease gradually in the anoxic and oxic zone, noting the high concentrations of nitrates and sulfates in the oxic zone. The sidestreams (BOD, NH4, and H2S) consume an amount of oxygen at 10% of the calculated amount to remove pollutants in the mainstream. The energy that was consumed from the sidestreams (BOD, NH4, and H2S) was 9.6% of the energy allocated to the mainstream, where 60% of the total percentage was due to ammonia. The percentage of alkali used to improve the nitrification process due to the ammonia in the sidestreams was 30% of the specified amount of ammonia in the mainstream.
The balance between 3 integrated sustainability requirements, including outlet quality, energy efficiency, and GHG emissions, should remain inside modern wastewater treatment plants. This novelty contributed to increasing the renewable energy in the plant by about 214 kilowatts and reducing the energy consumed in the plant by about 412 kilowatts, in addition to reducing the carbon footprint to 12%.

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