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In wastewater treatment processes, there are several parameters used as indications of water quality for checking the suitability of the treatments. These quality parameters can usually be determined by simple instruments rather than complete analyses, and they include pH, conductivity, dissolved oxygen, turbidity and total solids.
The pH is a measure of acidity or basicity of water, the normal neutral pH range is 6.5-8.5. As shown in Table1, the pH of the synthetic wastewater was 6.22 at 22.3Â°C; it was outside the range and indicated the pH of the wastewater was acidic. Therefore, the wastewater was abnormal in quality, and it was slightly corrosive. If the wastewater was discharged, it would affect or even kill the aquatic organisms. Neutralization is recommended to be conducted to treat the wastewater by addition of alkaline reagent.
Conductivity is the ability of solution to carry electric current, and it depends on ions concentration, ion mobility, ion valence and temperature. Most solutions of inorganic compounds are better conductor than those of organic compounds. According to the result, the synthetic wastewater had a high conductivity, 1796 mS/m, it represents the sample had large amount of inorganic compounds. There may be high amount of metal ions presented in the sample because of high conductivity. Chemical precipitation can be used to remove heavy metal ions, such as Zinc ion, Nickel ion, Copper ion, by the addition of lime/ caustic.
Chemical precipitation can be used as water softening to remove Calcium & Magnesium ions by addition of carbonate in hard water.
Furthermore, chemical oxidation can be utilized to remove iron & manganese ions.
According to result, Dissolved Oxygen of sample was 5.85 mg/ L at 22.3Â°C. DO is a criteria for removal of organic matter. In biological wastewater treatment processes, oxygen acts as terminal electron acceptor for obligate aerobes during activated sludge, nitrification and trickling filter. Oxygen is involved in microbial metabolism including the respiration and cell synthesis of microorganism.
DO is used to convert NH4+ into NO3- in nitrification, including Nitrosomonas: NH4+ + 3/2O2 à NO2- +2H++H2O+energy; followed by Nitrobacter: NO2-+1/2O2àNO3-+energy. In addition, DO of 7mg/L provide sufficient oxygen for the growth of aquatic organisms. The DO of sample was quite low, representing microbial growth in the wastewater.
The high turbidity of the synthetic wastewater sample represents there is high in suspended and colloidal matter and organic matter (e.g. clay, slit, finely divided matter, microorganisms). Physical and chemical wastewater treatments can be unitized to treat the wastewater. Some suspended particles can be removed by filtration, sedimentation and flocculation, or by chemical process, such as coagulation for very small suspended particle in colloidal form (0.003-1Î¼m). By addition of hydrated alum [Al3+(SO4).18H2O](Al3+) during coagulation Al3++H2Oà2AlOH2++H+, the coagulant can form precipitates trapping impurities, then the trapped impurities settle to the bottom of sedimentation tank.
Total Solids are substances left on the dish after evaporation of a solution and subsequent drying in oven at a defined temperature. TS include Total Suspended Solids (non-filterable solid) and Total Dissolved Solids (filterable solid). As shown in Table 3, TS of sample was high (59084mg/L). Some materials such as sesame, sludge and mud would have high TS. In this sample, 22% is TSS and 78% is TDS, most of the solids were filterable in the wastewater sample. Therefore, water quality parameters of the wastewater can be improved by filtration. According Chapter 358 Water Pollution Control Ordinance, if this wastewater was going to be discharged to Tolo harbor and port shelter water control zones in Hong Kong, the upper limit of suspended solids should be 30mg/L for flow at or below 1000m3/day, and 15mg/L for flow rate above 1000m3/day respectively. Therefore, the solid contents was too high for discharge, this wastewater should be re-treated before discharged.
For determination of Total Solid, the evaporation and ignition methods are reliable. On the other hand, for determination of volatile solids, some errors may be involved. It was because ammonium carbonate and some volatile organic compounds had already broken down under the high-temperature in the previous analysis.
In the measurement on total solid, the synthetic wastewater was measured by measuring cylinder before pouring in the evaporating dish. The problem may arise since some of the semi-solids may adhere on the measuring cylinder which may leads to weight loss. Therefore, proper mixing of semi-solid and liquid in the synthetic water is essential.
In measurement of total suspended solid, the semi-solid may adhere on the side of filter holder, so some of the solid may loss so as to affect the final result. Thus, synthetic wastewater should be poured at the centre of glass fiber filter.
Concerning on weighing of crucible, some sweat from hand may present on the surface of crucible which would affect the final weight of either sample or crucible. Forcep or gloves are recommended to use to minimize this error. Besides, water from atmosphere may absorb by crucible if the crucible was not dried improperly and sufficiently, so crucible can only leave from the dessicator when weighing. Different balance may have variation on weighing, so same balance should be used for weighing to reduce the error, i.e. using similar balance to weigh the crucible before and after drying.
Regarding the measurement of settleable solids, a longer time used for settling the solid, and a firmer and thinner layer will form. Error will be occurred if we settle the solid by different time period.
In order to obtain a more accurate result, first of all, prevent the probe of conduct meter from being dried, because of drying can damage the glass inside the probe. Then, avoid the suction of instrument when measuring turbidity and total solids. If the turbidity and total solids of sample are too high, the suction may easily happened. Thirdly, the concentration of sample should not too high, due the detection limit of instrument used in testing. In addition to, prevent contamination when carry the sample through different area.
Turbidity measurement makes use of the optical properties of water sample. There are several methods to determine the turbidity, different units are being used and eventually the different turbidity values. Besides, the wavelength used in different methods is not the same, the color of the particles in sample cause variance in turbidity. Some particles (e.g. organic material) strongly absorb incident light and prevent a significant portion of light from reaching the detector and result in a low turbidity values. The different particle size in sample may also causing impact to the turbidity. The larger particle scatter long wavelength more effectively than short wavelength.
In this experiment, pH, conductivity, dissolved oxygen, turbidity and total solids of the synthetic wastewater sample has been determined. The synthetic water had slightly acidic pH. According to the conductivity, it may contain some ions such as inorganic compounds like heavy metals. The high turbidity represented there were some suspended, colloidal matter and organic matter. Moreover, there may be microbial growth since the level of the dissolved oxygen was a little bit lower than normal level. For the solid contents, most of the solids in the sample are actually filterable. Therefore, filtration methods can be used to improve the water quality before discharge.
In conclusion, the water quality parameters reflected that the wastewater was not suitable for discharge according to Water Pollution Ordinance. Therefore, the synthetic wastewater should be well-treated before discharge.