Towards a rational design for sustainable urban drainage systems: understanding (bio)geochemical mechanisms for enhanced heavy metal immobilization in filters

Sustainable urban Drainage Systems (SuDS) have become an important approach for protection of natural watercourses from non-point sources of pollution. In particular, filtration based SuDS build on the concept of simple, low-cost technology that has been utilized in water treatment for over a century. While it is widely studied and acknowledged that filtration of polluted water through granular material is extremely effective, the inherent geochemical and biogeochemical mechanisms are complex and difficult to ascertain. This is especially true for SuDS filter drains as they have been less well studied. Therefore, this thesis set out to quantify heavy metal removal in gravel filter drains and investigate (bio)geochemical mechanisms responsible for metal immobilization. Determining specific mechanisms responsible for pollutant removal within SuDS provides data that can be used to enhance SuDS design and performance. First, the impact of engineered iron-oxide coatings on heavy metal removal rates were investigated. It was determined that unamended microgabbro gravel immobilized similar quantities of heavy metals to the engineered iron oxide coated gravel. Consequently, engineered iron-oxide coatings were not recommended for future research or use in SuDS systems. Analysis of the surface of microgabbro gravel revealed the surface minerals are weathering to clays, enhancing the gravels affinity for heavy metals naturally. Comparison of microgabbro with other lithologies demonstrated microgabbro displayed enhanced removal by 3-80%. Comparison of microgabbro gravels with and without weathered surfaces demonstrated the weathered surface enhanced metal removal by 20%. From this, it is recommended weathered microgabbro gravel be used in filtration based SuDS where immobilization of incoming heavy metals typical in surface water runoff is important. Following this, the contribution to metal immobilization due to biofilm growth in a gravel filter was examined. Through heavy metal breakthrough curves obtained from experimental flow cells with and without biofilm growth, it was determined that biofilm enhances heavy metal removal between 8-29%. Breakthrough curves were modelled with an advection diffusion equation. The model demonstrated heavy metal removal mechanisms within the column could be described effectively by a permanent loss term. Further, the typical microbial community found within biofilms collected from an urban filter drain was determined to be composed of over 70% cyanobacteria. However, when inoculated into two different lithologies of gravel, the biofilm community composition changed and was influenced by gravel lithology. Dolomite gravel retained 47% cyanobacteria dominance while microgabbro demonstrated 54% proteobacteria dominance. Despite variations in biofilm composition, heavy metal removal capacity and mechanisms were broadly similar between different biofilm types. An additional approach to determine effects of biofilm growth on porosity and flow patterns through a horizontal gravel flow cell was assessed with non-invasive magnetic resonance imaging (MRI). While a copper (Cu) tracer could be imaged within the gravel flow cell, the transport pathways were too complicated to model as the Cu does not follow a plug flow. Processing of 3D high resolution images determined the porosity of the gravel filter to be between 32-34%, in line with literature values for coarse grained dolomite gravel. Further post-processing allowed for localized biofouling to be analyzed. Highest concentration of biofilm growth in columns resulted from longer growth periods and exposure to light. Moreover, biofilms tended to grow closer to the inlet which typically offers a higher nutrient dose and in pore space regions close to the light source (both of which would be representative of the surface of a filter drain). Thus, MRI analysis of biofouling has important implications for filter drain design and efficiency through assessment of pore space blockage. Finally, the possibility of enhancing heavy metal removal in sand (another filter material common in SuDS) with nano zero-valent iron (nZVI) particles was considered. Metal breakthrough curves for column experiments indicate that use of 10% nZVI enhanced sand improved metal immobilization between 12-30% and successfully removed > 98% Cu and Pb. It is therefore believed that nZVI enhanced sand is a promising avenue of future research for areas prone to high heavy metal loads.