Chemical Pre-Treatment of Sewage - A Cost-Benefit Method
375
In Sweden, the first genuine chemical treatment plants came into operation for
the purpose of reducing phosphorus in 1968. By 1975 more than 500 treatment
plants were in full-scale operation (Ulmgren 1975).
Studies were done from the 1960s to compare different chemicals and to
determine optimal dosages and optimum pH values for the phosphorus removal
efficiency (Chung and Bhagat 1973; Evers 1973; Ulmgren 1975; Medboe et al.
1985; Gillberg et al. 1996) and also costs (Kumar and Clesceri 1973; Neis 1985;
0degaard and Karlsson 1994).
Aluminum, calcium, and iron compounds, with or without supplementary
additives, have been suggested for advanced treatment of secondary treated
effluents for phosphorus removal. The mechanisms of phosphorus removal
include precipitation, coagulation, and adsorption. The chemicals react with
orthophosphate and form a fine colloidal phosphate precipitate. At the same time -
during a hydrolysis process, in the presence of HC03• ions - the chemicals form
large aluminum and iron(III)-hydroxide flocs. These flocs coagulate the fine
colloidal phosphate precipitate as suspended solids, as well as adsorbing some of
the organic bound phosphorus (Evers 1973.) Through these mechanisms,
chemicals are also able to remove a significant part of the suspended solids and
organic matter as well.
A number of early reports were published also on coagulation and flocculation
of secondary effluents with different chemicals such as alum, ferric salts or lime
(Harris and Oda 1964; Malhotra et al. 1964; Rebhun et al. 1969; Rebhun and Streit
1974; Narkis et al. 1975). Ferric and aluminum salts act as coagulants through
their positively charged hydroxo complexes, while lime treatment at high pH
causes the formation of positively charged Mg(OHh and CaC03 precipitate. All
these reaction products of the chemicals, added with water constituents, act as
coagulants and flocculants, either through mutual neutralization, or adsorption and
bridging (Narkis et al. 1975; Hahn 1985). When using lime, CaC03 removes the
suspended solids and the easily coagulable large colloidal material. However,
stable and organic colloids are not affected by calcium carbonate. Mg(OH)z acts
as an adsorptive coagulant, providing a large adsorptive surface area and having a
positive electrostatic surface charge, and thus is also very efficient in coagulation
of stable organic colloids as well. The positive effect of Mg was observed
especially in alkaline media, where 25-30% of the organic matter is removed by
Mg(OH)z (Leentvaar and Rebhun 1981; Zotter and Licsk6 1992).
In many cases, coagulant aids such as anionic or cationic polymers, activated
silica, were used to improve the flocs. Rebhun et al. (1969) describe the use of
polyelectrolytes (cationic and nonionic) in combination with aluminum salts and
bentonitic clay as a tertiary treatment of municipal wastewater.
Similar experiments were done in Hungary in the 1970s (Libor et al. 1983). In
these researches a mixture of aluminum salts, activated bentonite and polymers
was used for coagulation-flocculation. Earlier, this combination was applied
successfully to treat wastewater from pulp mills in Sweden and Germany.
In Norway two thirds of the wastewater-treatment plants (built after the
beginning of the 1970s) apply chemical treatment, while half of them rely only on
375
In Sweden, the first genuine chemical treatment plants came into operation for
the purpose of reducing phosphorus in 1968. By 1975 more than 500 treatment
plants were in full-scale operation (Ulmgren 1975).
Studies were done from the 1960s to compare different chemicals and to
determine optimal dosages and optimum pH values for the phosphorus removal
efficiency (Chung and Bhagat 1973; Evers 1973; Ulmgren 1975; Medboe et al.
1985; Gillberg et al. 1996) and also costs (Kumar and Clesceri 1973; Neis 1985;
0degaard and Karlsson 1994).
Aluminum, calcium, and iron compounds, with or without supplementary
additives, have been suggested for advanced treatment of secondary treated
effluents for phosphorus removal. The mechanisms of phosphorus removal
include precipitation, coagulation, and adsorption. The chemicals react with
orthophosphate and form a fine colloidal phosphate precipitate. At the same time -
during a hydrolysis process, in the presence of HC03• ions - the chemicals form
large aluminum and iron(III)-hydroxide flocs. These flocs coagulate the fine
colloidal phosphate precipitate as suspended solids, as well as adsorbing some of
the organic bound phosphorus (Evers 1973.) Through these mechanisms,
chemicals are also able to remove a significant part of the suspended solids and
organic matter as well.
A number of early reports were published also on coagulation and flocculation
of secondary effluents with different chemicals such as alum, ferric salts or lime
(Harris and Oda 1964; Malhotra et al. 1964; Rebhun et al. 1969; Rebhun and Streit
1974; Narkis et al. 1975). Ferric and aluminum salts act as coagulants through
their positively charged hydroxo complexes, while lime treatment at high pH
causes the formation of positively charged Mg(OHh and CaC03 precipitate. All
these reaction products of the chemicals, added with water constituents, act as
coagulants and flocculants, either through mutual neutralization, or adsorption and
bridging (Narkis et al. 1975; Hahn 1985). When using lime, CaC03 removes the
suspended solids and the easily coagulable large colloidal material. However,
stable and organic colloids are not affected by calcium carbonate. Mg(OH)z acts
as an adsorptive coagulant, providing a large adsorptive surface area and having a
positive electrostatic surface charge, and thus is also very efficient in coagulation
of stable organic colloids as well. The positive effect of Mg was observed
especially in alkaline media, where 25-30% of the organic matter is removed by
Mg(OH)z (Leentvaar and Rebhun 1981; Zotter and Licsk6 1992).
In many cases, coagulant aids such as anionic or cationic polymers, activated
silica, were used to improve the flocs. Rebhun et al. (1969) describe the use of
polyelectrolytes (cationic and nonionic) in combination with aluminum salts and
bentonitic clay as a tertiary treatment of municipal wastewater.
Similar experiments were done in Hungary in the 1970s (Libor et al. 1983). In
these researches a mixture of aluminum salts, activated bentonite and polymers
was used for coagulation-flocculation. Earlier, this combination was applied
successfully to treat wastewater from pulp mills in Sweden and Germany.
In Norway two thirds of the wastewater-treatment plants (built after the
beginning of the 1970s) apply chemical treatment, while half of them rely only on