Design Model for COD Removal in Constructed Wetlands Based on Biofilm Activity

Current design criteria for free-water-surface constructed wetlands are based on either empirical relationships or first-order reaction rates and do not emphasize the microbial activity. This study was conducted to evaluate the role of biofilm bacteria present in free-water-surface constructed wetland beds in the removal of organic matter. A kinetic model incorporating the biofilm kinetics and dispersion number was proposed to predict chemical oxygen demand removal in free-water-surface constructed wetlands. The model parameters were determined from laboratory experiments and from the literature. The proposed kinetic model satisfactorily predicted chemical oxygen demand removal efficiencies in a pilot-scale constructed wetland unit located in the tropics. A design chart and a design example based on the kinetic model are given.     

An integrated electro-chemical and natural treatment system for industrial water pollution control.

Experiments were conducted to test the feasibility of applying an integrated electro-chemical (EC) and natural treatment system for treatment of some industrial wastewaters. The EC process was found to be very effective in removing lead, a model heavy metal from some wastewaters. Within 20 minutes of operation time, 5 to 10 A of electric current and specific surface area of electrode of 46.51 m2/m3, the lead concentrations in the wastewaters were reduced from 35-100 mg/l to less than 1 mg/l. Based on a kinetic model developed from the experimental data, the important parameters for the EC process were found to be electric current, specific surface area of electrode, and operation time. From scanning electron microscopic and X-ray diffractometric (XRD) analysis, the EC sludge samples were found to compose mainly of maghemite (Fe2O3), magnetite (Fe3O4), and laurionite (PbClOH), suitable for disposal to secure landfills. Two pilot-scale constructed wetlands (CW) in series, a model natural treatment system, were employed to treat wastewaters of an industrial estate in Thailand. At organic loading rates of 57-140 kg BOD/hectare-year, these constructed wetlands were able to reduce BOD from 90 to 4 mg/l, while suspended solids, total nitrogen and total phosphorus were reduced from 100 to 10 mg/l, 24 to 4.6 mg/l and 7 to 1.5 mg/l, respectively, during the summer season. These results demonstrated technical feasibility of CW in removing organic and other pollutants contained in this industrial wastewater.     

Airborne Polycyclic Aromatic Hydrocarbons (PAH) in Bangkok Urban Air: Part II. Level and Distribution

The gas-particle partitioning and particle size distributions of airborne PAH in Bangkok urban air were investigated using an 8 stage size fractionating cascade impactor (Andersen “low volume” sampler) and a downstream XAD-2 adsorbent tube for sample collection. Nine PAH classified as carcinogenic and co-carcinogenic compounds-Pyrene (PYR), Benz(a)Anthracene (BaA), Benzo(e)Pyrene (BeP), Dibenz(a,c)Anthracene (DBacA), Benzo(k)Fluoranthene (BkF), Benzo(a)Pyrene (BaP), Dibenz(a,h)Anthracene (DBahA), Benzo(ghi)Perylene (BghiP) and Trimethylcholanthrene (3MC) - were quantified. The lower molecular weight (MW) PAH such as PYR, BeP and BaA were present mainly in the gaseous phase (80%, 40% and 24%, respectively) while higher MW compounds were present almost totally in the particulate fraction. The results show that 30%–60% of each PAH by mass were found on particles smaller than 0.43 μm and more than 70% on particles with diameter less than 2.1 μm. In addition, the relationship between the particle/gas partition coefficient (K _p ) and the sub-cooled liquid vapor pressure (p°_L) was also determined to describe the gas-particle partitioning of those PAH compounds in a tropical atmosphere.     

Wastewater treatment in a deep aeration tank

Pilot-scale experiments were conducted to investigate the performance of a deep aeration tank (DAT) (10 m deep) treating a high-strength synthetic wastewater and the DAT biokinetics. At the mean cell residence of 2 days or at the food to microorganisms ratio of 1.41 and less, the DAT reactor, operating continuously as completely mixed and without cellular recycle, removed more than 95% of the chemical oxygen demand of the wastewater. The treatment kinetics were observed to follow the Lawrence and McCarty's models in which the values of the kinetic constants Y, k_d, k and K₃ for this particular operating condition were found to be 0.53, 0.085, 9.25 day^?1 and 259 mg 1^?1, respectively. The results of solid separation by flotation, using the hydrostatic pressure developed in the DAT reactor, were satisfactory. However, better flotation results can be expected with the proper design and operation of the flotation tank.     

Study on chlorine disinfection of pond effluent

Chlorine disinfection experiments were conducted to investigate the die-off patterns of the fecal coliforms when present in waste stabilization pond effluents containing from 100 to 400 mg l⁻¹ algae. The fecal coliform inactivation was observed to occur at two rates, i.e. an initial rapid kill followed by a slower kill. The magnitude of inactivation was found to be proportional to the initial chlorine dose and contact time and inversely proportional to the algal concentration. A mathematical model was developed to predict the fecal coliform survival ratio during the chlorination of the pond effluent. When compared with the experimental data, the predicted results had a correlation coefficient of 0.981.     

Media configuration and recirculation of upflow anaerobic floating filter for piggery wastewater treatment

An upflow anaerobic floating filter media (UAFF) reactor was applied to the treatment of synthetic and real piggery wastewater. The effect of media configuration and internal recirculation on the system performance was studied. In the first experiment, three-UAFF reactors filled with different media, i.e., polypropylene beads, sponge cubes and coconut fiber were continuously fed with synthetic wastewater at upflow velocity of 0.04 m h⁻¹. The COD removal efficiency in the reactor filled with sponge cubes was highest at 90%, whereas the others filled with polypropylene beads and coconut fibers with lower specific surface area were about 80%. In the second experiment, three-UAFF reactors with sponge were applied to treat real piggery wastewater. COD removal efficiencies were found to be about 80% and methane production rate of 0.26 l l _r ⁻¹d⁻¹. The system performance could be slightly improved by 10% when applying internal recirculation. A sludge blanket (60–70% of total biomass) plays an important role in the system when applied to the treatment of piggery wastewater containing high suspended solid concentration.     

Integrated faecal sludge treatment and recycling through constructed wetlands and sunflower plant irrigation.

Faecal sludge (FS) from the on-site sanitation systems is a nutrient-rich source but can contain high concentrations of toxic metals and chemicals and infectious micro-organisms. The study employed 3 vertical-flow CW units, each with a dimension of 5 x 5 x 0.65 m (width x length x media depth) and planted with cattails (Typha augustifolia). At the solid loading rate of 250 kg total solids (TS)/m(2).yr and a 6-day percolate impoundment, the CW system could achieve chemical oxygen demand (COD), TS and total Kjeldahl nitrogen (TKN) removal efficiencies in the range of 80-96%. A solid layer of about 80 cm was found accumulated on the CW bed surface after operating the CW units for 7 years, but no clogging problem has been observed. The CW percolate was applied to 16 irrigation sunflower plant (Helianthus annuus) plots, each with a dimension of 4.5 x 4.5 m (width x length). In the study, tap water was mixed with 20%, 80% and 100% of the CW percolate at the application rate of 7.5 mm/day. Based on a 1-year data in which 3 crops of plantation were experimented, the contents of Zn, Mn and Cu in soil of the experimental plots were found to increase with increase in CW percolate ratios. In a plot with 100% of CW percolate irrigation, the maximum Zn, Mn and Cu concentrations of 5.0, 12.3 and 2.5 mg/kg, respectively, were detected in the percolate-fed soil, whereas no accumulation of heavy metals in the plant tissues (i.e. leaves, stems and flowers) of the sunflower were detected. The highest plant biomass yield and oil content of 1000 kg/ha and 35%, respectively, were obtained from the plots fed with 20% or 50% of the CW percolate.     

Application of chemical precipitation for piggery wastewater treatment.

Several series of experiments were conducted to investigate the treatment of piggery wastewater using chemical precipitation (CP) where various types of coagulants such as aluminium sulfate (Al2(SO4)3), poly aluminium chloride (PAC), ferric chloride (FeCl3), ferric sulfate (Fe2(SO4)3), ferrous sulfate (FeSO4) and ferrous chloride (FeCl2) were used. Throughout the experiments, CP was found to achieve high removal efficiencies for organic compounds and nutrients (nitrogen and phosphorus) from the piggery wastewater. Experimental results showed the optimal doses of FeCl3, Fe2(SO4)3, FeCl2 and FeSO4 was 2.0 g/L, while 0.31 g/L and 2.5 g/L were the optimum dose for PAC and Al2(SO4)3, respectively. The pH range 4-5 resulted in the best performance to all coagulants except FeCl2 and FeSO4, whose optimum pH were more than 6. Percentage removal efficiencies for COD were in the ranges of 70-80%, 90-95% for SS, 80-90% for organic-N and TP. Those removal efficiencies were achieved within 5 min of operation. Three times of repetition in CP resulted in higher removal efficiencies for COD, SS and colour up to 74%, 99% and 94% respectively, in which Al2(SO4)3 was used as the coagulant. Removal efficiencies of various water quality parameters in a continuously operated reactor were similar to those of the batch experiments. Biodegradable ratios (BOD5/COD) increased up to 65% after the application of CP.     

Nitrogen mass balance and microbial analysis of constructed wetlands treating municipal landfill leachate.

Experiments were conducted to investigate the feasibility of applying constructed wetlands (CW) to treat a sanitary landfill leachate containing high nitrogen (TN) and bacterial contents. Under the tropical conditions (temperature of about 30 degres C), the CW units operating at a hydraulic retention time (HRT) of 8 days yielded the best treatment efficiencies with BOD5 removal of 91%, TN removal of 96%, total and fecal coliforms (TC and FC) removal of more than 99%. Cadmium removal in the in the SFCW bed was found to be 99.7%. Mass balance analysis, based on TN contents of the plant biomass and dissolved oxygen (DO) and oxidation - reduction potential (ORP) values, suggested that 88% of the input TN were uptaken by the plant biomass. Fluorescence in situ hybridization (FISH) results revealed the predominance of bacteria including the heterotrophic and autotrophic bacteria responsible for BOD5 removal. Nitrifying bacteria was not found to be present in the SSFCW beds.     

Treatment of septage in constructed wetlands in tropical climate: lessons learnt from seven years of operation.

In tropical regions, where most of the developing countries are located, septic tanks and other onsite sanitation systems are the predominant form of storage and pre-treatment of excreta and wastewater, generating septage and other types of sludges. The septage is disposed of untreated, mainly due to lack of affordable treatment options. This study presents lessons that have been learned from the operation of pilotscale constructed wetlands (CWs) for septage treatment since 1997. The experiments have been conducted by using three CW units planted with narrow-leave cattails (Typha augustifolia) and operating in a vertical-flow mode. Based on the experimental results, it can be suggested that the optimum solids loading rate be 250 kg TS/m2 yr and 6-day percolate impoundment. At these operational conditions, the removal efficiencies of CW units treating septage at the range of 80-96% for COD, TS and TKN were achieved. The biosolid accumulated on the CW units to a depth of 80 cm has never been removed during 7 years of operation, but bed permeability remained unimpaired. The biosolid contains viable helminth eggs below critical limit of sludge quality standards for agricultural use. Subject to local conditions, the suggested operational criteria should be reassessed at the full-scale implementation.