Solids Loading Capacity of Stormwater Biomedia for Estimating Biofilter Longevity

Biofilter, bioretention, and raingardens all rely on biomedia to remove solids, nutrients, and metals and to support viable plant growth. In response to the growing interest in low impact development (LID), many studies of biomedia have focused on pollutant removal performance. However, some fundamental questions central to the biomedia longevity and maintenance requirements remain unclear, which often impedes a sound comparison of different practices and sometimes leads to improper biofilter design and deployment. Once biomedia is clogged, incoming stormwater runoff is either bypassed or ponded without treatment and no water quality improvement is attained. Therefore it is critical to study the biofilter service time and the needed maintenance scheme in addition to the performance. To this end, this paper investigated UrbanGreen (UG) biomedia loading and longevity with natural stormwater solids (less than 500 μm) instead of silica material. A series of iterative loading tests with natural stormwater solids were carried out to clog select biomedia. To evaluate the restorability of biomedia infiltration capacities, each test was followed by surficial biomedia replacement to mimic standard maintenance practices. The average natural stormwater sediments concentration was controlled at an approximate level of 200 mg/L. The biomedia depth was 24 in. and constant infiltration rate of 100 in./hr was maintained throughout the testing. The experiment was completed after 12 cycles. The average surface loading rate for each cycle is 2.3 lb/ft^2 with standard deviation of 0.49 lb/ft^2. The cumulative loading capacities warrant UG biomedia a minimum service life of 5-10 years depending on the specific site loading conditions and biofilter sizing. The results provide valuable information that can be used to predict maintenance intervals and service life of biomedia treatment systems.