Chemical Pre-Treatment of Sewage - A Cost-Benefit Method
387
7 Conclusions
Chemical pretreatment significantly reduces the organic matter and phosphorus
contents in the primary settling tank, providing a low load for the activated sludge
stage of WWTP. Depending on the quality of the raw sewage, and type and
dosage of coagulants, the removal rates of organic content are in the range of 6080% as CODcr and 50-65% as BODs. The residual CODcr and BODs should be
reduced to 130-180 and 80-120 mgr 1 concentration range, respectively. The
necessary coagulant [aluminum and iron(III)-salts] dose depends on the type of
coagulant and the quality of the raw sewage. The type and concentration of
different organic compounds, especially the colloid stabilizing matter, play a
significant role. With the two investigated WWTPs, significantly different
coagulant doses were required for organic content removal. This means that there
are no general instructions; the optimal coagulant dosage has to be determined
plant by plant. The minimum effective AI3+ and Fe3+ dosages changed in the range
of 20-40 and 20-60 mgr 1 concentration, respectively. The higher dosages were
required at Nl WWTP.
In the same coagulation dosage ranges exceeded 90% removal of phosphorus,
while the residual concentration complied with the required limits (2.0 mgr 1 at Nl
and 1.0 mgr 1 atBI WWTPs).
Chemical pretreatment can provide a significant improvement in the removal of
phosphorus and organic content at low capital costs (Nl) and acceptable OMR
costs. With Bl WWTP, chemical pretreatment reduced the phosphorus
concentration and significantly lowered the organic load of the activated sludge
stage, resulting in stable nitrification in the high load system at low OMR cost.
References
Andersson B, Aspegren H, Nyberg U, Jansen J, la Cour, 0degaard H (1992)
Evaluation of pre-precipitation in a wastewater treatment system for extended
nutrient removal, In: Chemical water and wastewater treatment II. - Proc of the 5 th
Gothenburg Symp 1992, Springer, Berlin Heidelberg New York, pp 341-355
Chung SK, Bhagat SK (1973) Wastewater treatment properties. Water Sewage Works
120(2):54-58
Evers RH (1973) Advanced waste-water treatment techniques for removing nitrogen
and phosphorus, Water Sewage Works 120: Ref. No. RI5 ... RI9 and RI34 ... R137.
Gillberg L, Nilsson D, Akesson M (1996) The influence of pH when precipitating
orthophosphate with aluminum and iron salts, In: Chemical water and wastewater
treatment IV. - Proc of the 7 th Gothenburg Symp 1992, Springer, Berlin Heidelberg
New York, pp 95-105
Hahn HH (1985) Physical and chemical aspects of coagulation in water technology In:
Chemical water and wastewater treatment - practical experience and new concepts.
Proc 1st Gothenburg Symp 1984. Gustav Fischer, Stuttgart, pp 1-28
387
7 Conclusions
Chemical pretreatment significantly reduces the organic matter and phosphorus
contents in the primary settling tank, providing a low load for the activated sludge
stage of WWTP. Depending on the quality of the raw sewage, and type and
dosage of coagulants, the removal rates of organic content are in the range of 6080% as CODcr and 50-65% as BODs. The residual CODcr and BODs should be
reduced to 130-180 and 80-120 mgr 1 concentration range, respectively. The
necessary coagulant [aluminum and iron(III)-salts] dose depends on the type of
coagulant and the quality of the raw sewage. The type and concentration of
different organic compounds, especially the colloid stabilizing matter, play a
significant role. With the two investigated WWTPs, significantly different
coagulant doses were required for organic content removal. This means that there
are no general instructions; the optimal coagulant dosage has to be determined
plant by plant. The minimum effective AI3+ and Fe3+ dosages changed in the range
of 20-40 and 20-60 mgr 1 concentration, respectively. The higher dosages were
required at Nl WWTP.
In the same coagulation dosage ranges exceeded 90% removal of phosphorus,
while the residual concentration complied with the required limits (2.0 mgr 1 at Nl
and 1.0 mgr 1 atBI WWTPs).
Chemical pretreatment can provide a significant improvement in the removal of
phosphorus and organic content at low capital costs (Nl) and acceptable OMR
costs. With Bl WWTP, chemical pretreatment reduced the phosphorus
concentration and significantly lowered the organic load of the activated sludge
stage, resulting in stable nitrification in the high load system at low OMR cost.
References
Andersson B, Aspegren H, Nyberg U, Jansen J, la Cour, 0degaard H (1992)
Evaluation of pre-precipitation in a wastewater treatment system for extended
nutrient removal, In: Chemical water and wastewater treatment II. - Proc of the 5 th
Gothenburg Symp 1992, Springer, Berlin Heidelberg New York, pp 341-355
Chung SK, Bhagat SK (1973) Wastewater treatment properties. Water Sewage Works
120(2):54-58
Evers RH (1973) Advanced waste-water treatment techniques for removing nitrogen
and phosphorus, Water Sewage Works 120: Ref. No. RI5 ... RI9 and RI34 ... R137.
Gillberg L, Nilsson D, Akesson M (1996) The influence of pH when precipitating
orthophosphate with aluminum and iron salts, In: Chemical water and wastewater
treatment IV. - Proc of the 7 th Gothenburg Symp 1992, Springer, Berlin Heidelberg
New York, pp 95-105
Hahn HH (1985) Physical and chemical aspects of coagulation in water technology In:
Chemical water and wastewater treatment - practical experience and new concepts.
Proc 1st Gothenburg Symp 1984. Gustav Fischer, Stuttgart, pp 1-28