Organic polymers as solo coagulants in municipal wastewater treatment

Abstract

As the demand for water continues to grow, so does the volume of wastewater generated. Wastewater has constituents that pollute receiving waterbodies hence its treatment is very essential. The aim of wastewater treatment is to reduce the concentration of polluting substances before its release to the environment. To this end, many processes (physical, biological, chemical) are employed in the treatment of wastewater depending on the desired/required effluent water quality. Chemical treatment processes are applied when wastewater constituents (such as phosphorus, dissolved solids, etc.) that cannot be removed by physical or biological treatments are present. Pollutants present in wastewater carry negative surface charges and are rather in a stable state suspension. Chemical coagulation involves the destabilization of the suspension by the addition of positively charged coagulants.

The most commonly used coagulants are metal salts (IC) of aluminium and iron. Although these salts of multivalent metals can remove a broad range of contaminants (e.g. dissolved, colloidal, etc.), they are known to be effective in high doses and at specific pH, produce large amount of sludge, consume alkalinity of water, and hinder the use of sludge as soil amendment due to the strong bond they form with phosphate. Although studies have indicated that organic coagulants (OC) can compensate for some of the shortcomings of metal salts during coagulation, there is still a lack of knowledge regarding the potential of OC to replace IC in municipal wastewater treatment. Thus, the aim of this thesis work was to examine the effectiveness of OC (synthetic and semi-natural) as solo precipitation agents on municipal wastewater treatment in comparison to the most commonly used IC under laboratory conditions.

The jar-test methodology was used to evaluate and compare the effectiveness of IC and OC of varying molecular weight and charge density in reducing the turbidity, suspended solids (SS), total phosphorus (tot-P) and phosphate phosphorus (PO4-P), organic matter, nitrogen and metal concentrations from primary and secondary wastewater samples. A total of 12 coagulants were tested, these include ferric sulphate and polyaluminium chloride (metal salts), polydiallyldimethylammonium chloride (polyDADMACs) and polyamines (synthetic polymers) of varying molecular weight and charge density, tannin, chitosan and starch-based products (semi-natural polymers). These were tested on real municipal wastewater samples (Taskila wastewater plant, Oulu – Finland); 12 coagulants for the secondary sedimentation stage and 5 coagulants for the primary sedimentation stage of treatments.

Dosage requirements depended on the water quality to be treated. In general, IC required higher dosages and among the organic products, those with higher MW required the lowest dosages. Results showed that OC can perform as well as IC provided that the right type of coagulant at the right dosage is added. At the optimum dosages identified, IC achieved purification efficiencies of up to 53% of SS, 66% of tot P, 84% of PO4-P and 70% of organic matter in secondary wastewater samples. Comparatively, OC achieved removal of up to 72% of SS, 76% of tot-P, 93% of PO4-P, 88% of organic matter. The results for the treatment of primary wastewater samples with selected coagulants showed removal of up to 93% of SS, 86% of tot-P, 96% of PO4-P and 70% of organic matter by the IC whereas the OC removed up to 86% of SS, 62% of tot-P, 97% of PO4-P, 91% of organic matter.