Over the last few years, there has been an increase in the volume of wastewater, which requires increased removal of biogenic elements in order to meet the requirements for acceptable values of wastewater pollution indicators. Wastewater treatment plants are developing new solutions also due to the reduction of investment and operating costs. Companies are looking for methods of highly efficient removal of nitrogen, phosphorus and carbon compounds, which will improve the quality of treated wastewater and also reduce electricity consumption. One of the primary processes used at treatment plants is nitrification, which is an aerobic process and has high energy consumption. Another process is denitrification, which involves the reduction of nitrification products under anoxic conditions to gaseous nitrogen and its removal from wastewater.
One of the proposed technologies to reduce the cost of wastewater treatment is to use a shortcut nitrification process in the main sequence. The shortcut nitrification process involves carrying out only the first phase of nitrification, called nitritation, and inhibiting the second phase of nitrification, called nitratation. The inhibition of the second phase of nitrification consists in causing the inhibition of Nitite Oxidizing Bacteria (NOB), which are more sensitive to many factors than the Ammonium Oxidizing Bacteria (AOB) responsible for carrying out the first phase.
The first solution that can be applied is to choose the right sludge retention time (SRT). Nitrifying bacteria are autotrophs that bind inorganic carbon for mass gain and multiplication. They obtain the energy to bind 1 carbon atom by oxidizing 35 NH3 molecules and 100 nitrite ions. Due to the method of obtaining carbon-containing compounds, the biomass gain of autotrophic bacteria is small, however AOB show a slightly larger gain compared to NOB so the selection of an sludge retention time, greater than the minimum for AOB bacteria and less than the minimum for NOB bacteria, will ensure inhibition of phase II bacteria.
Further inhibitors of NOB bacteria are free ammonia and free nitric acid (III). Based on literature data, the value below which complete nitrification occurs is FA < 0.1mg NH3/dm3. On the other hand, above this constant, a progressive inhibition of nitratation can be observed. On the other hand, a decrease in nitritation activity is noticeable only after exceeding 10 mg NH3/dm3. In contrast, free nitric acid (III) causes inhibition of second phase nitrifiers at concentrations as low as 0.02mg HNO2/dm3, and first phase nitrifiers at concentrations of about 0.1 to 0.2 mg HNO2/dm3. Therefore, by maintaining proper conditions in the reactor, we can control the concentration of these compounds and bring about the inhibition of selected bacteria.
Promoting the growth of first phase bacteria is also possible by regulating dissolved oxygen concentration as a result of differences in oxygen affinity constants for AOB and NOB bacteria. Nitritation is dependent on the presence and activity of the AOB bacteria, and due to the different affinity constants for these bacteria, an appropriate oxygen concentration is sufficient to inhibit the growth of the NOB bacteria. In addition, the second phase microorganisms show much less resistance to lower oxygen concentrations regardless of their fixed affinities for oxygen.
The combination of shortcut nitrification and denitrification processes seems to be a good solution that can be used to improve the energy balance of the treatment plant. In unit terms, it allows for a 25% reduction in oxygen demand and a 40% reduction in the amount of organic compounds supplied to the reactor in relation to the nitrification and denitrification processes. Application of the solution proposed in the article directly translates into lower operating costs of the wastewater treatment plant.
The SNIT project consists of four stages, three of which will be implemented in Poland and one in Norway. In the first stage, research will be conducted on the process of culturing phase I nitrifiers on leachate from dewatering sludge after the fermentation process. At the second stage, methods of NOB removal from activated sludge in the main sewage line will be investigated. The elimination of these bacteria should lead to a decrease in the consumption of organic compounds and oxygen. The proposed activities will contribute to nitrate (III) remaining in the treated wastewater, therefore it will be necessary to eliminate these compounds from the wastewater (Stage 3 of the project). One of the solutions will be the culture of phase II nitrifying bacteria (NOB) on media immersed in the nitrification chamber (IFAS – Integrated Fixed Film Activated Sludge), and the second will be the application of a denitrifying sand filter after the secondary settling tanks. The aim of the last phase, carried out by a foreign partner, is to evaluate a new method of sewage sludge disintegration and to increase the biogas potential by adding fish sludge from fish farms to the sludge being digested.
