mentation (6 hours or less), rapid sand filtration 

 (3 gal/sq ft/min or less) and disinfection with 

 chlorine (without consideration to concentration 

 or form of chlorine residual). A wide variety of 

 modifications of this basic treatment process are in 

 use for removing various impurities or altering 

 quality characteristics, but we have arbitrarily 

 excluded these modifications in our deliberations 

 because of the difficulty in deciding where to stop 

 in considering the many modifications and elabora- 

 tions of the basic process. 



introduction 



THE NATIONAL Technical Advisory Sub- 

 committee on Public Water Supplies has 

 found it necessary to make some rather arbitrary 

 decisions in order to proceed with its task of de- 

 veloping raw water quality criteria for public water 

 supplies. Because public water supplies commonly 

 involve processing of the raw water to improve its 

 quality before distributing it to consumers, and 

 because treatment processes exist which can, at a 

 price, convert almost any water including sea 

 water and grossly polluted fresh water into a pot- 

 able product, it is necessary to consider the type 

 of treatment in any discussion of raw water quality 

 criteria for public water supplies. 



We have adopted as the considered treatment 

 the most common processes in use in this country 

 in their simplest form for the treatment of surface 

 waters for public use. This may include coagula- 

 tion (less than about 50 ppm alum, ferric sulfate, 

 or copperas with alkali addition as necessary but 

 without coagulant aids or activated carbon), sedi- 



We have listed two types of criteria defined as 

 follows : 



(a) Permissible criteria. — Those character- 

 istics and concentrations of substances in 

 raw surface waters which will allow the 

 production of a safe, clear, potable, aes- 

 thetically pleasing, and acceptable public 

 water supply which meets the Umits of 

 Drinking Water Standards (10) after 

 treatment. This treatment may include, 

 but will not include more than, the proc- 

 esses described above. 



(b) Desirable criteria. — Those characteristics 

 and concentrations of substances in the 

 raw surface waters which represent high- 

 quality water in all respects for use as pub- 

 lic water supplies. Water meeting tiiese 

 criteria can be treated in the defined plants 

 with greater factors of safety or at less cost 

 than is possible with waters meeting per- 

 missible criteria. 



Several words used in the table and in the text 

 require explanation in order to convey the Sub- 

 committee's intent: 



Narrative. — The presence of this word in the 

 table indicates that the Subcommittee could not 

 arrive at a single numerical value which would be 

 applicable throughout the country for all condi- 

 tions. Where this word appears, the reader is 

 directed to the appropriate explanatory text. 



Absent. — ^The most sensitive analytical pro- 

 cedure in Standard Methods (9) (or other ap- 

 proved procedure) does not show the presence of 

 the subject constituent. 



Virtually absent. — This terminology implies 

 that the substance is present in very low concen- 

 trations and is used where the substance is not 

 objectionable in these barely detectable concen- 

 trations. 



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