Slow sand filtration: recent research and application perspectives

N. J. D. Graham* and M. R. Collins

ABSTRACT

Slow sand filtration (SSF) has been widely used in the field of water treatment for over 100 years and while important features of the process have been studied in detail, many aspects of the process remain poorly understood. In addition, modifications to the process and pre-treatment methods are required to enhance SSF performance to meet more demanding treated water quality objectives and reduced costs. This paper highlights some of these aspects and refers to recent research that has investigated the nature and role of the biomass in the process, the benefits of applying surface fabric layers, pre-ozonation and GAC sub-layers, and the development of a deterministic-type mathematical model to simulate the SSF process. In all cases, the research has provided useful insights but much futher work is required to consolidate the findings and bring benefits to the application of SSF in practice.

Keywords Slow sand filtration; biofiltration; fabric layers; pre-oxidation; GAC amendments; process modelling

INTRODUCTION

The process of slow sand filtration (SSF) has been widely used in the field of water treatment for over 100 years but while important features of the process have been studied in detail, many aspects of the process remain poorly understood. This is partly explained by the greater attention that has given to the more widely applied rapid filtration process, but also because of the inherent complexity of SSF. Much is known of the ecology and fundamental dynamics of the filter process in qualitative terms (eg. common biological species and their interactions), but a comprehensive quantitative description of the process remains to be established.

In the 1990's slow sand filtration was the subject of a series of international conferences (Graham, 1988; Collins and Graham, 1994; Graham and Collins, 1996), professional guidance manuals (AWWA, 1991; ASCE, 1991), and literature reviews (eg. Lambert and Graham, 1995), which provided an extensive body of reference material and details of ongoing research studies. Since then, there has continued to be a steady flow of publications in the scientific literature concerned with various aspects of SSF research and plant operation (e.g. Gimbel et al., 2006). Broadly, these have been in four areas of SSF application, namely: drinking water treatment where the SSF is the principal treatment step (eg. for small community water supplies after rudimentary pre-treatment); drinking water treatment where the SSF is a secondary process following extensive prior treatment (Rachwal et al., 1996); the tertiary treatment of wastewaters (eg. Muhammad and Morris Hooke, 2003), particularly for effluent re-use; horticultural applications, such as the disinfection of recirculating nutrient solutions (Garibaldi et al., 2003). Only developments in drinking water applications will be described in this paper.

The key areas of general interest in SSF include pre-treatment, process mechanisms, treatment performance, modelling, and process enhancement. Associated with these are specific limitations or gaps in our current knowledge, such as the following:

i) a comprehensive, quantitative description of the fundamental process mechanisms;

ii) the relationship between the influent water quality and the nature of the SSF schmutzdecke;

iii) the removal of natural and synthetic organic substances;

iv) predicting filter run time;

v) methods of increasing filtration rates and filter run time;

vi) enhanced cleaning technologies.

Aspects of SSF performance that continue to be the subject of study are the nature of the biomass (in the schmutzdecke and filter bed), the mechanisms of treatment, the impact and role of animals, the removal of pathogens and specific substances (e.g. pesticides, pharmaceutical and endocrine disrupting compounds), and the benefits of covering and media amendments. In view of the broad range of the above topics, particular aspects of previous, and potentially future, research will be referred to here. These are: the nature and development of the biomass in SSF; the application of fabric layers to support the schmutzdecke; the effect of ozone as a pre-treatment for SSF; the incorporation of GAC sub-layers for enhanced organics removal; the development of a deterministic process model.

BIOMASS

The high degree of water treatment achieved by SSFs is partly explained by the slow filtration rate (0.1 - 0.3 m/h) and fine effective size of the sand (0.1 - 0.3 mm), but is also attributed to biological processes in the layer of material that accumulates above the sand surface (schmutzdecke) and within the upper layers of the sand bed. The schmutzdecke is also believed to be primarily responsible for the progressive increase of head loss observed during filter operation. Despite its current and historical importance as a water treatment process, the fundamental biological composition and mechanisms affecting water purification and head loss development during SSF runs remain poorly defined (Haig et al., 2011). Microbial biomass in the schmutzdecke and filter sand bed has been quantified using a range of microbiological methods (e.g. Duncan, 1988; Yordanov et al., 1996), but the different approaches used to measure biomass concentrations and the inconsistent units and sampling intervals adopted have confounded inter-study comparisons of biomass development and behaviour during SSF. Most reports are of single measurements of the net biomass production at the end of a filter run, prior to cleaning, and these emphasise the significant variability apparent in schmutzdecke and sand biomass accumulation in operational slow sand filters. The collection of representative samples of schmutzdecke and sand material during filter operation is difficult in practice, and the lack of a simple routine method for measuring microbial biomass are probable reasons for the limited amount of field-scale investigation of the biological mechanisms of SSF. Detailed analyses of biomass growth in the schmutzdecke and within the sand bed during filter operation would improve understanding of the complex and fundamental interactions between the biological and physico-chemical processes operating in the filter and enable the development of mechanistic models for SSF operational management (e.g. prediction of head loss rate, run time, frequency of sand cleaning and renewal).

One recent study has considered the development of microbial...