The effect of high hydraulic loading rate on the removal efficiency of a quadruple media filter for tertiary wastewater treatment

Abstract

It is well known that filtration removal efficiency falls with an increase in flow rate; however, there is limited supporting experimental data on how removal efficiency changes for filters with multiple layers of media and for wastewater filtration, a practice that is becoming more common. Furthermore, information is not available on the characteristics of particles that are removed at different flow rates. Here, a quadruple media filter was operated at hydraulic loading rates (HLRs) between 5 and 60 mh with subsequent measurement of total suspended solids, turbidity and particle size distribution (PSD). Samples were collected from the filter influent, effluent and also from between media layers. Pressure changes across the filter layers were also measured. The solids removal efficiency of the filter varied inversely with the increase in filtration rate. However, the multiple media layers reduced the negative impact of increased HLR in comparison to a single media filter. High filtration rates were shown to transport solids, such that particle retention and headloss development was distributed across the entire depth of the multi-media filter. There was also a progressive decrease in the suspension particle size leaving each of the filter layers. The particle hydrodynamic force simulation was consistent with the changes in measured PSD through the filter layers.

The impact of filter bed depth and solids loading using a multimedia filter

“…A decrease in media size with depth results in an increase in collector surface area with depth. This observation illustrated the improvement in solids retention that could be achieved by replacing a single media filter by a multimedia filter of the same total depth [12]. For both the anthracite and quadruple filter of 0.32 m depth, each suspended solid influent concentration increase of 10 mgL -1 reduced the removal efficiency by an average factor of 0.9 while the solids retention increased by a factor of 1.2.…”

“…Secondary effluent from the STWs discharge well was pumped to a mixed holding tank from where the feed was transferred to the filter rig (more detail in Ncube et al [12]). The quadruple media pressure filter design was adapted using the same media layers as used in a commercial filter system (FilterClear, BluewaterBio, UK).…”

“…Significant benefits in particle removal and headloss can be realised through using new filters designs. This includes application of new filter media such as compressible crumb rubber [10,11] and multi-media filters that utilise up to four different media of different particle size and density [12].…”

Consequences of pH change on wastewater depth filtration using a multimedia filter

“…In this research, a quadruple granular media filter adapted from a FilterClear pressure filtration system (BluewaterBio, UK) was used to investigate removal performance of each media at different pH. The pilot filter consisted of four columns (clear acrylic Perspex, 74 mm internal diameter and 700 mm height) connected in series, each containing 100 mm media depth of either anthracite, flint, alumina and magnetite respectively; the detailed description of the filter equipment is found in Ncube et al (2016).…”

“…Attractive forces are conventionally represented as negative hence the minus sign on the second half of the term in equation 1. Apart from the adhesive forces, there are other forces acting on particles as the water flows through the porous media, such as the hydrodynamic forces associated with a scouring effect on deposited solids (Bai and Tien, 1997;Ncube et al, 2016). However, such forces were assumed not to vary at any pH investigated since the filter hydraulic loading rate was maintained constant.…”