Optimizing suspended solids removal during chemical treatment of humic and low turbidity water : purification of peat extraction runoff
1University of Oulu, Faculty of Technology, Environmental Engineering
|Online Access:||PDF Full Text (PDF, 6.6 MB)|
|Persistent link:|| http://urn.fi/URN:NBN:fi:oulu-201805312008
|Publish Date:|| 2018-06-01
|Thesis type:||Master's thesis (tech)
Over the years, methods have been developed for the treatment of peat extraction runoff water such as treatment wetlands, sedimentation ponds and chemical treatment. In Finland, authorities consider chemical treatment one of the best available technologies for purifying peat extraction runoff. However, significant variation in purification efficiency has been observed in different sites specially during periods of low suspended solid concentration in the inflowing runoff water.
The main objective of this thesis was to evaluate practical solutions for the optimisation of processes i.e. coagulation and flocculation, in order to improve the efficiency and reliability of the chemical treatment of the typically humic and low turbidity waters discharging from peat extraction sites. The aim was to investigate chemicals (including novel and hybrid products) and measures which had the potential to improve the removal of suspended solids while also identifying suitable alternatives that maintained the pH of the treat-ed water inside acceptable level (pH 5). Coagulants tested were ferric sulphate, aluminum sulphate, polyaluminum chloride (conc. 45-50%), polyaluminum chloride (conc. 30-40%), polyaluminum chloride (recovered from industrial process), tannin-based biopolymer, in-house blend of ferric sulphate and polyDADMAC, in-house blend of aluminum sulphate and polyDADMAC and commercially available blend of poly-DADMAC and aluminum chloride.
Jar test methodology was employed and water samples from two different locations were treated in laboratory scale experiments. Research methodology was elaborated to i) identify the optimum dosage of tested coagulants for the treatment of the different water samples, ii) Investigate the influence of process parameters (pH, mixing, and sediment addition) on load removal efficiency and iii) Evaluate the settling properties of flocs formed by different coagulants. Purification efficiency was evaluated based on the percentage removal of suspended solids (SS), dissolved organic carbon (DOC), total nitrogen (tot-N), total phosphorous (tot-P), phosphate phosphorous (PO4-P) and ammonium (NH4) from the water samples being purified.
In general, all coagulants and blends tested achieved high purification efficiencies in the treatment of both water samples. Removal efficiencies achieved were for water sample 1: 50-80% DOC, 30-60% tot-N, 78-100% tot-P and 30-80% SS. For water sample 2: 65-80% DOC, 73-83% tot-N, 67-87% tot-P and 57-85% SS. Ferric sulphate and its blends presented higher removal of DOC (ca 78%) compared to aluminum sulphate and its blends (50-60%). Ferric sulphate performed well also in the removal of nitrogen (ca 59%) but aluminum sulphate and its blends achieved best removal rates for ammonium (ca 52%). Although there was good load removal by monomers and their blends, the pH of treated water was affected with values close 4 ob-served. In the other hand, pre-hydrolysed coagulants performed well achieving removal rates among the highest and without affecting the water pH.
Influence of mixing on the performance of ferric sulphate and polyaluminum chloride (conc. 45-50%) was evaluated. While mixing conditions had a significant influence on the performance of ferric sulphate, removal of turbidity and colour by polyaluminum chloride was not affected by the variations applied to mixing pa-rameters. Regarding the settling characteristics of flocs formed, ferric sulphate presented the fasted sedimentation rate while polyaluminum chloride (conc. 45-50%) was the best performing coagulant achieving the best turbidity removal after 25 min of sedimentation. The addition of recovered sediments from a chemical treatment facility during the purification process was conducted in an attempt to improve turbidity removal. No measurable effect was observed in the purification rates achieved when sediments were added.
In conclusion, polyaluminium chloride products (conc. 45-50%) were found to be suitable solutions to improve purification efficiency and reliability of the chemical treatment of peat extraction runoff water. Furthermore, these products do not affect the pH of the water significantly avoiding the need for pH neutralisation.
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