Process modification and low-cost intervention of an old sewage treatment plant to improve biological nutrient removal

Typically, municipal wastewater is collected from cities and is treated at centralized sewage treatment plants (STPs), which are then discharged to nearby surface water bodies.  A city placed downstream picks up this water from the water body for its usage.  As the distance between the cities gets closer with increasing population and urbanization, treating the wastewater in STPs to a much higher treatment standard becomes necessary.  A 2–5-day interval is assumed between the point of discharge and the offtake point into the next town /city downstream.  This interval has now shortened to a few hours, especially in the developing world. Another issue is about the life of the STP and the reigning norms for treatment quality.  Many of these centralized STPs have a typical life of about 25-30 years and are designed to achieve specific effluent discharge standards formulated when designing them. However, these effluent discharge standards have been made more stringent over the years, requiring municipalities to improve the treated wastewater quality every decade. Therefore, to meet new discharge standards, there is a need to modify or remodel these old STPs.  This can be achieved either by process modifications or retrofitting existing STPs with modern machinery.  However, retrofitting an existing STP can be an expensive proposition; also, it is time-consuming in terms of design, approval, construction, and commissioning.  Thus, it is better to modify the treatment process while retaining the plant capacity to its original design.  Mathematical models are useful tools for optimization and process modifications of STPs.

In this work, a plant-wide BioWin - activated sludge mathematical model was developed for the 55 MLD treatment unit in the KC valley sewage treatment plant in Karnataka, India. The model was calibrated using STOWA protocol and validated with experimental field results. Eight process equations, including fifty state variables, were solved for the modeling. In order to improve plant performance, a modification in the treatment process was proposed. The proposal was to introduce anoxic/anaerobic zones in between the existing aeration zones in the plant to improve simultaneous nitrification-denitrification and total dissolved phosphorous (TDP) removal.  Several sensitivity analyses were carried out to identify the optimized operating conditions for this process modification. The dissolved oxygen and mixed liquor suspended solid concentrations in the aeration zones were varied from 2.5 to 4 mg/L and 2500 to 4500 mg/L, respectively, until the optimized conditions were achieved. The modifications were then implemented in the 55 MLD unit with minimal intervention and without shutting down the plant. The plant performance, as predicted by the model, improved after the modification.  The effluent's total nitrogen and TDP values were reduced from 20 mg/L to 8 mg/L and 3.5 mg/L to 0.9 mg/L before and after the modification. All other parameters effluent parameters were also within the standard discharge limits. Also, a total saving of 2,60,2490 USD in capital costs and 1,48,910 USD in operational costs was achieved by this modification.