Chapter 22 - Drinking water treatment (116 pages)

Introduction
1.Surface water treatment systems
1.1.Raw water storage
1.2.Water intake
1.2.1.Water intake design
1.2.2.Pre-treatment at the raw water intake
1.3.Pre-oxidation
1.3.1.Aeration
1.3.2.Chemical oxidation
1.4.Clarification
1.4.1.Filtration without reagents
1.4.2.Coagulation, followed by filtration
1.4.3.Complete clarification treatment
1.5.Polishing: removal of organic matter and micro-pollutants
1.5.1.General
1.5.2.Polishing «tools»
1.5.3.Application examples
1.6.Disinfection
1.6.1.Definition
1.6.2.Bactericidal effect - Residual effect
1.6.3.General conditions required for satisfactory disinfection
1.6.4.Conditions governing the application of the various disinfectants
1.7.Membrane processes
1.7.1.Nanofiltration
1.7.2.The Cristal process
1.7.3.The extended Cristal process
1.7.4.Other cases involving the use of membranes in drinking water treatment
1.8.Problems associated with algae and zooplankton
1.8.1.Eliminating planktonic micro-algae
1.8.2.Eliminating algal metabolites
1.8.3.Problems associated with zooplankton and protozoan cysts
1.9.Artificial aquifer recharge
1.10.Sludge processing
1.10.1.Nature of the sludge
1.10.2.Coagulation over filters
1.10.3.Complete treatment
2.Specific treatments
2.1.Iron removal
2.1.1.The natural states of iron
2.1.2.Physical-chemical iron removal
2.1.3.Biological removal of iron - The Ferazur process
2.2.Manganese removal
2.2.1.Natural states
2.2.2.Physical-chemical removal of manganese
2.2.3.Biological removal of manganese - The Mangazur process
2.3.Ammonia removal
2.3.1.Physical-chemical processes
2.3.2.Biological removal of ammonia (nitrification)
2.3.3.Biological treatment of water containing ammonia, iron and/or manganese
2.4.Removal of nitrates
2.4.1.Physical-chemical processes - The Azurion process
2.4.2.Biological denitrification processes
2.4.3.Compared advantages of the two nitrate removal techniques
2.5.H₂S removal
2.5.1.Physical removal by stripping
2.5.2.Chemical processes
2.5.3.Biological processes
2.5.4.Conclusion
2.6.Fluorination and defluorination
2.6.1.Fluorination
2.6.2.Fluorine removal
2.7.Arsenic removal
2.7.1.General
2.7.2.The GEH process
2.8.Other metalloids (or non-metals)
2.8.1.Antimony removal
2.8.2.Boron removal
2.8.3.Bromine: the problem of bromates
2.8.4.Selenium removal
2.9.Heavy metals removal
2.9.1.Behaviour within the systems
2.9.2.Special case of the European standards
2.9.3.The lead problem
2.10.Radioactivity
2.11.Salinity removal
2.12.Organic matter removal
2.12.1.NOM - Colour
2.12.2.Tastes and odours
2.12.3.Organic micro-pollutants
2.12.4.Special case of chlorinated solvents
3.Changes to the carbonate balance
3.1.Neutralisation and/or remineralisation
3.1.1.Neutralising carbonic attack
3.1.2.Remineralisation
3.1.3.Reagent consumption
3.2.Carbonate removal and/or softening
3.2.1.Softening through carbonate removal
3.2.2.Softening over resin
3.2.3.Electro-carbonate removal
4.Small standard units
4.1.UCD (Degrémont compact unit) (figur 48)
4.2.Pulsapak
4.3.UEP (pesticide removal unit)
4.4.Ultrasource
5.Renovating (or «refurbishing») existing plants
5.1.Increasing output and/or improving treatment
5.2.Coming into line with new water quality standards
5.3.Conclusion

Introduction

The object of this chapter is to show how «treatment train» arrangements combining a range of technologies can be used to process the most difficult fresh water source to render it potable, bearing in mind that brackish water or seawater desalination was reviewed in chapter 15 § 4.2.
The reader is invited to refer to chapters 3 and 4 for an outline of the theoretical principles pertaining to the physical-chemical and biological treatment processes utilised and also to the chapters describing the various Degrémont technologies, in particular: settling tanks - flotation units (chapter 10), filters (chapter 13), membranes (chapter 15) and oxidation-disinfection (chapter 17).
Typically, the available freshwater resources are distinguished by three major categories (see chap. 2):

surface water: this category of water is frequently described by the degree and type of suspended solids (including organic materials, colloidal particles, and algae concentrations that are subject to rapid variation); dissolved organic matter (natural or artificial, color); pathogenic organisms (viruses, bacteria, protozoan parasites...) and sometimes particular minerals such as heavy metals;
deep groundwater, on the other hand, normally contains no suspended solids, pathogenic organisms nor OM other than NOM; however, groundwater frequently contains reduced compounds such as Fe(II), Mn(II), NH₄ and even toxic minerals such as As, Se, F..., radioactive elements, or groundwater sources may simply be too hard or soft, too saline (C -, SO₄^2-), or contain too many nitrates. It should be noted that each of these elements require specialised treatment strategies unlike surface water sources, which typically call for a more generalised treatment approach (clarification, polishing, disinfection), resulting in the general layout of this chapter;
groundwater «under the influence of surface water», intermediate amongst the first two categories, this group includes very shallow groundwater sources, water in karstic systems..., and is often clear but susceptible to rapid deterioration from turbidity and all the other contaminants found in surface waters and deep groundwater sources.

Note: a fourth category can also be identified: wastewater recycled for human consumption, usually indirectly (injected into aquifers, into water reservoirs to be processed into drinking water...), bearing in mind that this type of water contains certain pollutants from categories 1 and 2 and that synthetic pollutants play a very important role. One also needs to recall that the main treatments applicable to this category of water tend to be those described in chapters 11 and 24 (biological treatment) or 15 (membrane separation).
Therefore, to render water potable, there exists an entire range of technologies of which practically none are truly specific to any of the pollutant types referred to above. This is illustrated by table 1 where the X's denote the processes that play a major role in the elimination of a particular type of contaminant and where x's denote a process that contributes to said elimination but as a secondary property.
As already emphasised, the processes and technologies outlined in table 1 vary widely in terms of cost (capital investment and operational) as well as operating limitations and performance. Consequently, defining an optimum treatment train continues to form part of the «craft» of the Design Engineer with many examples highlighted in this chapter.
Note: although residuals handling (sludge, saturated adsorbants, membrane backwash waste...), with or without preliminary treatment, is not covered in detail within this chapter, it remains an integral component of the overall cost and must therefore be considered when selecting the optimal treatment design.
This table highlights the noteworthy effectiveness of clarification and/or desalination membranes, explaining the increasing tendency of water treatment specialists to recommend them either to replace conventional treatment strategies (filtration of karstic water, disinfection), or to supplement conventional treatment as part of a more advanced system to process more heavily contaminated water.
Depending on the properties of the water to be treated and the effectiveness of a given process, the water specialist will thus be able to group each elementary «stage» into what is commonly referred as a «treatment system».