flowers
hi freind
thanks for ur replay, its a simple information and i know it. please if u have any more about this subject.
i need ebooks and lectures and any websites dealing with this subject
thanks alot
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waste water treatment:
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3.2.2 Primary treatment
The objective of primary treatment is the removal of settleable organic and inorganic solids by sedimentation, and the removal of materials that will float (scum) by skimming. Approximately 25 to 50% of the incoming biochemical oxygen demand (BOD5), 50 to 70% of the total suspended solids (SS), and 65% of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorus, and heavy metals associated with solids are also removed during primary sedimentation but colloidal and dissolved constituents are not affected. The effluent from primary sedimentation units is referred to as primary effluent. Table 12 provides information on primary effluent from three sewage treatment plants in California along with data on the raw wastewaters.
Table 12: QUALITY OF RAW WASTEWATER AND PRIMARY EFFLUENT AT SELECTED TREATMENT PLANTS IN CALIFORNIA
Quality parameters (mg/l, except as otherwise indicated)
City of Davis
San Diego
Los Angeles County Joint Plant
Raw wastewater
Primary effluent
Raw wastewater
Primary effluent
Raw wastewater
Primary effluent
Biochemical oxygen demand,BOD5
112
73
184
134
-
204
Total organic carbon
63.8
40.6
64.8
52.3
-
-
Suspended solids
185
72
200
109
-
219
Total nitrogen
43.4
34.7
-
-
-
-
NH3-N
35.6
26.2
21.0
20.0
-
39.5
NO-N
0
0
-
-
-
-
Org-N
7.8
8.5
-
-
-
14.9
Total phosphorus
-
7.5
-
10.2
-
11.2
Ortho-P
-
7.5
11.2
-
pH (unit)
7.7
-
7.3
7.3
-
-
Cations:
Ca
-
-
-
-
78.8
-
Mg
-
-
-
-
25.6
-
Na
-
-
-
-
357
359
K
-
-
-
-
19
19
Anions:
SO4
-
160
270
Cl
-
120
397
Electrical conductivity, dS/m
2.52
2.34
2.19
-
Total dissolved solids
-
-
829
821
1404
1406
Soluble sodium percentage, %
-
-
70.3
Sodium adsorption ratio
-
-
-
-
8.85
6.8
Boron (B)
-
-
-
-
1.68
1.5
Alkalinity
(CaCO3
)
-
-
-
322
332
Hardness (CaCO3)
-
-
265
Source: Asano and Tchobanoglous (1987)
In many industrialized countries, primary treatment is the minimum level of preapplication treatment required for wastewater irrigation. It may be considered sufficient treatment if the wastewater is used to irrigate crops that are not consumed by humans or to irrigate orchards, vineyards, and some processed food crops. However, to prevent potential nuisance conditions in storage or flow-equalizing reservoirs, some form of secondary treatment is normally requiredinthese countries, even in the case of non-food crop irrigation. It may be possible to use at least a portion of primary effluent for irrigation if off-line storage is provided.
Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units. Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units.
In large sewage treatment plants (> 7600 m3/d in the US), primary sludge is most commonly processed biologically by anaerobic digestion. In the digestion process, anaerobic and facultative bacteria metabolize the organic material in sludge (see Example 3), thereby reducing the volume requiring ultimate disposal, making the sludge stable (nonputrescible) and improving its dewatering characteristics. Digestion is carried out in covered tanks (anaerobic digesters), typically 7 to 14 m deep. The residence time in a digester may vary from a minimum of about 10 days for high-rate digesters (well-mixed and heated) to 60 days or more in standard-rate digesters. Gas containing about 60 to 65% methane is produced during digestion and can be recovered as an energy source. In small sewage treatment plants, sludge is processed in a variety of ways including: aerobic digestion, storage in sludge lagoons, direct application to sludge drying beds, in-process storage (as in stabilization ponds), and land application.
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3.2.5 Disinfection
Disinfection normally involves the injection of a chlorine solution at the head end of a chlorine contact basin. The chlorine dosage depends upon the strength of the wastewater and other factors, but dosages of 5 to 15 mg/l are common. Ozone and ultra violet (uv) irradiation can also be used for disinfection but these methods of disinfection are not in common use. Chlorine contact basins are usually rectangular channels, with baffles to prevent short-circuiting, designed to provide a contact time of about 30 minutes. However, to meet advanced wastewater treatment requirements, a chlorine contact time of as long as 120 minutes is sometimes required for specific irrigation uses of reclaimed wastewater. The bactericidal effects of chlorine and other disinfectants are dependent upon pH, contact time, organic content, and effluent temperature.
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3.2.7 Reliability of conventional and advanced wastewater treatment
Wastewater reclamation and reuse systems should contain both design and operational requirements necessary to ensure reliability of treatment. Reliability features such as alarm systems, standby power supplies, treatment process duplications, emergency storage or disposal of inadequately treated wastewater, monitoring devices, and automatic controllers are important. From a public health standpoint, provisions for adequate and reliabile disinfection are the most essential features of the advanced wastewater treatment process. Where disinfection is required, several reliability features must be incorporated into the system to ensure uninterrupted chlorine feed.
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