Preparation Of Coagulant From Fly Ash And Its Application In Coagulation-Constructed Wetland

The secondary biochemical treatment processes were mostly adopted in China to treat municipal wastewater. However, because of their high primary investment and operating cost, long construction period and large floor area, a new flexible wastewater treatment process, named combined chemically enhanced primary treatment (CEPT)-constructed wetlands treatment system, was presented in this study, aiming at the development of an effective process with less capital investment and operating cost available in medium and small cities. To reduce the chemical dosage cost for the CEPT process, fly ash (FA) produced from a local power plant was firstly utilized to develop an efficient compound coagulant. The effluent from wastewater coagulation unit was then applied to feed the designed subsurface-flow constructed wetland (SSFCW) to further remove the remained pollutants. Meanwhile, a Na₂CO₃ solution was slowly added to the acid leaching fly ash solution to prepare the polyaluminum ferric sulfate(PAFS), which was applied to coagulation of industrial wastewater.Cooperating with acid leachant concentration increasing, the enhancement of leaching temperature could greatly promote aluminum and ferrum leaching from FA. Reacting at boiling temperature under atmospheric pressure, the converting efficiencies of Al and Fe could achieve 10% and 33%,respectively. Effective composition of prepared fly ash coagulant were 19.3g/L Al₂(SO₄)₃ and 7.5g/L Fe₂(SO₄)₃. When fly ash coagulant was dosed at a rate of 4.0ml/L, sewage treatment effect was equivalent with that of commercial coagulant. However, the volume of sludge was still large, since 6kg fly ash particles would turn into chemical sludge when disposing per cubic metre sewage. The addition of Cl- or F- to fly ash didn't improve acid leachability. Acid leachability of Al was improved after FA roasting with Na₂CO₃ at high temperature. According to mass ratio of Na₂CO₃ to FA 0.06, roast the mixture at 805℃for 1 h, then leach the clinker by 4 mol/L ([H⁺]) sulfuric acid at boiling temperature for 0.5h with the water vapor condensed. Being cooled, complex coagulant was obtained, which contained Al₂(SO₄)₃ of 32.7g/L and Fe₂(SO₄)₃ of 7.1 g/L. When this complex coagulant was dosed at a rate of 1ml/L, sewage treatment effect was remarkably superior to that of commercial coagulant with equal dosage, and the removal rates of COD, TP and SS could achieve more than 64%, 91% and 93%, respectively. The coagulation effluent TP and SS had already met the level IB of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002).Through researches done on affection to coagulation performance of PAFS by checking various Al/Fe mol ratios, Na₂CO₃ concentration, pH of PAFS, Al+Fe concentration and types of base solution, it was proved a successful condition when Na₂CO₃ of 100g/L was slowly added to the acid solution until the pH of the acid solution raised to 1.1¹.2. The result showed that [Al,Fe]_a decreased as pH of PAFS raised while [Al+Fe]_b and [Al+Fe]_c increased. For PAFS with pH of 1.1¹.2, the [Al,Fe]_a species was the main component, 57.06%, with 5.58% of [Al+Fe]_b and 37.36% of [Al+Fe]_c. The change of pH of PAFS with aging time to a lower value was in agreement with the transformation of Al and Fe species of oligomers to high polymers. PAFS was composed of OH-Al complexes and OH-Fe complexes.PAFS had shown a high coagulation effect, superior to that of PAC for dairy wastewater treatment at the same dosage. The optimum coagulation pH range of PAFS is 6⁹. After sedimentation period of 15min, removal efficiencies of COD and SS by this type of coagulant reached 63.9% and 94.4%, respectively.When the coagulation effluent was further treated in the following SSFCW system with hydraulic loading 0.03⁰.10m³/(m²Â·d), COD loading 5.62¹8.11g/(m²Â·d), hydraulic retention time(HRT) 6.73¹.95d,COD could be removed up to 64%⁷7% with the final effluent COD less than 60 mg/L, which met the level IB of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002). Wetlands working in series could enhance ammonia nitrogen removal. With hydraulic loading 0.05m³/(m²Â·d), ammonia nitrogen loading 2.156 g/(m²Â·d), HRT 3.97d, effluent ammonia nitrogen was 40.46mg/L, with removal rate 11.97%, while total nitrogen(TN) removal rate was 15.44%, with effluent TN 46.80mg/L. In this lab-scale experiment, SSFCW system with gravel bed presented phosphorus release phenomena, with influent TP 0.3⁰.5mg/L while effluent TP about 1.0mg/L. However the effluent TP was still far below the levelâ…¡of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002).