Gravel roughing filtration

out-of-sequence; upcoming book section

This section provides a basic overview for using gravel roughing filtration as a low-cost, scalable pretreatment method for removing sediment and particulates from a water source prior to other unit processes such as biological slow sand filtration, carbon adsorption, disinfection using chlorine or ultraviolet light (UV), etc.

The purpose of roughing filtration is to remove suspended solid material or turbidity from water. Turbidity impairs subsequent treatment steps, for example by clogging slow sand filters, fouling carbon adsorbents, reacting with disinfectants such as chlorine, and blocking transmission of UV light. Turbid water also appears cloudy or murky, and may impart unpleasant taste and/or odor.

Gravel roughing filters can reduce source water turbidity by 60% to 90%, or more [1, 2]. They can also achieve one- to two- log reduction in fecal coliforms [1]. This level of bacterial removal is not sufficient to meet drinking water standards. However, this does provide performance assistance to subsequent treatment units such as slow sand filters or disinfection processes for achieving microbiological water quality requirements.

This section is not intended as a comprehensive discussion of roughing filtration pretreatment methods for drinking water. Please see the references listed at the end for more in-depth treatment of these and related topics. This section summarizes the most common configurations used in low-resource settings and provides an overview of design and operational guidelines. The section ends with design specifications for gravel roughing prefilters we have developed and used based on engineering guidelines and practical experience in treatment systems that utilize biologically active slow sand filtration for control of microbial pathogens and biochar adsorption for removal of harmful trace chemicals.