Civil Engineering Reference
In-Depth Information
CHAPTER 23
Residuals Management
INTRODUCTION
In the process of removing undesirable constituents from raw water, a variety of waste
products, known as residuals, is generated. Among the undesirable constituents are
sand and silt, organics in solution, suspended material, ions that cause hardness, total
dissolved solids, nitrates, arsenic, radionuclides, bacteria and other organisms, and
naturally occurring and synthetic organic matter. The most commonly used treatment
processes that remove these materials and subsequently produce a residual are chemi-
cal coagulation, lime-soda softening, sedimentation, removal of iron and manganese,
taste and odor control, filter backwashing, membrane separation, ion exchange, and
granular-activated carbon (GAC) adsorption.
The residuals may be discharged nearly continuously, as from clarifiers, or infre-
quently, from plain (no sludge collection equipment) settling basins. The residuals may
contain clay, silt, sand, carbon, chemical precipitates, bacteria and other organisms and
organic substances. The composition of residuals from a treatment plant treating sur-
face water is likely to vary daily, seasonally, and annually, as raw-water quality changes
occur. Residuals from one plant may be significantly different from residuals at a
nearby plant that uses the same raw-water source, because of differences in treatment
technique and chemical types and dosages.
CHARACTERIZATION OF RESIDUALS
Types of Residuals
Sludges
Semisolid residuals produced from mechanical water settling or clarification
processes (such as screening or presedimentation), in addition to those produced from
the clarification of water that has been chemically preconditioned, are typically referred
to as sludges.
Coagulant / Polymeric Sludges.
Chemical coagulation and subsequent flocculation are
widely used water treatment processes for removing clay, silt, dissolved or colloidal
organic material, microscopic organisms, and colloidal metallic hydroxides. Aluminum
sulfate (alum) is the most widely used coagulant, although iron salts—ferric chloride,
ferrous sulfate, and ferric sulfate—also are used as coagulants. Coagulation sludges
742
