Balance between water quality and water quantity based on retention time in a biofilter

Abstract

Urbanization has led to increased impermeable surfaces, disrupting the natural hydrological cycle and resulting in more frequent urban flooding and pollution. There is an urgent need for cost-efficient and effective stormwater management solutions to improve stormwater quality and facilitate its reuse. FieldFactors' BlueBloqs system integrates stormwater collection, water treatment in a biofilter, and aquifer recharge. The BlueBloqs system's biofilters are soil-plant systems with a submerged zone. Before being used to recharge an aquifer, the effluent from a biofilter must meet specific water quality standards. Improvements in water quality within the biofilter are achieved mainly through static optimization (changing media materials, the depth of layers, and plant species) and dynamic optimization (controlling hydraulic behavior). Some research found that retention time plays a crucial role in the operation of a biofilter, influencing both the quality and quantity of the effluent. The central challenge lies in operating the biofilter's retention time to maximize water recharge while ensuring optimal water quality.

The objective of this research is to determine the optimal retention time for the biofilter to achieve the best water quality while maximizing the total amount of water to be recharged by controlling the pump. This thesis includes water quality measurement and water quantity simulation. In the water quality part, the performance of the biofilter at different retention time (2, 6, 16, and 24 hours) is evaluated in terms of the removal efficiency of turbidity, nutrients (nitrate and phosphorus), dissolved and particulate metals (Mn, Ca, Mg, Ba, Zn), UV254 and DOC with two different influent types (surface water and stormwater). In the water quantity part, Cromvliet Park, with an 8000 m2 collection area, was modelled using Python and SWMM to operate the biofilter with various retention time under one year of rainfall data (total rainfall 731mm). This modelling was done to calculate the total water volume available for recharge. The sections on water quality and quantity were integrated by controlling and evaluating the biofilter based on the retention time.

The results indicate that the efficiency of the biofilter's pollutant removal is influenced by several factors. Turbidity (40-90%) is effectively removed by the filtration in the biofilter. Nitrate removal efficiency was 12-49% with short retention times (2-6 hours) but fluctuated with longer durations (16-24 hours), decreasing to -84-13% and then changing to -6-0%. Measurements of dissolved oxygen in the effluent indicated that an increase in nitrate removal efficiency due to nitrification happened with longer retention time. For dissolved zinc, removal efficiency ranged from 10-60% with stormwater influent and 0-100% with surface water because of the adsorption competition of metals. Combined with the water quantity calculations performed in SWMM, it is advisable to set the optimal retention time for the biofilter at 4–6 hours. This duration allows for the maximization of water reuse while maintaining high nitrate removal efficiency.