Polishing low-biodegradable and saline industrial effluent in a full-scale horizontal subsurface flow constructed wetland: evaluation of bio-treatability and predictive power of kinetic models

Abstract

This study evaluates the bio-treatability performance and kinetic models of full-scale horizontal subsurface flow constructed wetland used for the tertiary treatment of composite industrial effluent characterized by high-salt content ranging from 5830 to 10,400 µS/cm and biochemical oxygen demand (BOD₅): chemical oxygen demand (COD) ratio below 0.2. The wetland vegetated with Phragmites australis was operated in a semi-arid climate under an average hydraulic loading rate of 63?mm/d. The results of a 4-year operation calculated based on the concentration of pollutants showed that the average removal efficiency of COD, BOD_5, and total suspended solids (TSS) were 17.5, 5.1, and 11.2%, respectively. The system reduced up to 6.5?±?0.7% of electrical conductivity presenting poor phyto-desalination potential without considering the contribution of evapotranspiration in water balance in contrast to satisfying performance for heavy metals reduction. The comparison of the kinetics of organic matter removal obtained by the first-order and Monod models paired with continuous stirred-tank reactor and plug flow regime showed that Monod-plug flow model provided the best fit with the constants of 2.01?g COD/m²·d and 0.3014?g BOD₅/m²·d with the best correlation coefficient of 0.610 and 0.968 between the predicted and measured concentrations, respectively. The low kinetic rates indicate that the process is capable of effluent polishing instead of purification due to the presence of organic compounds recalcitrant to biodegradation and a high level of salinity.     

Kinetics of pollutant removal from domestic wastewater in a tropical horizontal subsurface flow constructed wetland system: Effects of hydraulic loading rate

The treatment capacity of constructed wetlands is expected to be high in tropical areas because of the warm temperatures and the associated higher rates of microbial activity. A pilot scale horizontal subsurface flow constructed wetland system filled with river sand and planted with Phragmites vallatoria (L.) Veldkamp was set up in the southern part of Vietnam to assess the treatment capacity and the removal rate kinetics under tropical conditions. The system received municipal wastewater at four hydraulic loading rates (HLRs) of 31, 62, 104 and 146 mm day^?1. Removals of TSS, BOD₅ and COD were efficient at all HLRs with mean removal rates of 86–95%, 65–83% and 57–84%, respectively. Removals of N and P decreased with HLRs and were: NH₄-N 0–91%; TN 16–84% and TP 72–99%. First-order area-based removal rate constants (k, m year^?1) estimated from sampling along the length of the wetland from inlet to outlet at the four HLRs were in the range of 25–95 (BOD₅), 22–30 (COD), 31–115 (TSS), 5–24 (TN and TKN) and 41–84 (TP) at background concentrations (C*) of 5, 10, 0, 1.5 and 0 mg L^?1, respectively. The estimated k-values should not be used for design purposes, as site-specific differences and stochastic variability can be high. However, the study shows that domestic wastewater can be treated in horizontal subsurface flow constructed wetland systems to meet even the most stringent Vietnamese standards for discharge into surface waters.     

Generating “Tide” in Pilot‐Scale Constructed Wetlands to Enhance Agricultural Wastewater Treatment

The operation of tidal flow was studied using a pilot‐scale system treating high strength piggery wastewater. Located on a farm in Staffordshire, UK, the system consisted of five wetland treatment stages vegetated with common reeds of Phragmites australis. Wastewater samples were collected from the inlet and outlet of each stage and analyzed for BOD₅, COD, NH₄‐N, NO₃‐N, NO₂‐N, SS, PO₄‐P and pH. Average hydraulic and organic loadings on the system were 0.12 m³/m² d and 240 g BOD/m² d, respectively, which is considerably higher than the typical loadings on conventional subsurface flow systems. On average, BOD₅ and COD were reduced by 82 % and 80 % from initial concentrations of 2000 mg/L and 2750 mg/L, respectively, across the whole system. The first‐order kinetics constant for BOD₅ removal (K_BOD in m/d) in this tidal flow system is approximately 2.5 times the rate constant obtainable in a typical horizontal flow system, demonstrating a more efficient removal of organic matter in tidal flow wetlands. The overall efficiency of the system was found to increase with time before stabilizing towards the end of a start‐up period. Straight‐line correlations were established between the loading and removal of BOD₅ and COD. Contributions by individual stages to the overall treatment were analyzed. SEM images of wetland media demonstrated the formation of biofilms and microbial activities inside the matrices of the wetland system, which accounted for the degradations of organic pollutants.     

Electricity from landfill leachate using microbial fuel cells: Comparison with a biological aerated filter

Four experimental columns were employed in this study to investigate their performance under wastewater treatment conditions. One column was set-up as a biological aerated filter and the remaining three were set-up as microbial fuel cells (MFCs), two of which were connected to an external load whereas the third was left open circuit. The performance of the columns under several flow rates and leachate strengths was studied in terms of BOD₅ removal efficiencies and electricity generation, when a fixed resistive load was connected. Results obtained demonstrated that it is possible to generate electricity and simultaneously treat landfill leachate in MFC columns. Energy generation in MFC columns improved with increasing flow rates from 24 to 192 mL/h, while BOD₅ removal efficiency levels reached a maximum at 48 mL/h and dropped to relatively low values at higher flow rates. The maximum removal efficiencies were obtained at a loading rate of 0.81 kg BOD₅/m³ d for columns C1, C2 and C4 and 1.81 kg BOD₅/m³ d for column C3. Electrical output levels and BOD₅ concentrations at the MFC columns showed a linear relationship, which allows the system to be used as a BOD₅ sensor. Part of the BOD removal was not associated with power generation and was attributed to the presence of alternative end terminal electron acceptors and volatilisation. The MFC columns could reach the same or even higher removal efficiencies than those from the biological aerated filter with the advantage of producing energy and saving cost of aeration. To the best of the authors’ knowledge, this is the first study that compares the MFC technology with other conventional treatment systems for removing pollutants from wastewater.     

Treatment of pig manure liquid digestate in horizontal flow constructed wetlands: Effect of aeration

With the rapid development of scaled anaerobic digestion of pig manure, the generation of liquid anaerobic digestate exceeds the farmland loading capacity, causing serious environmental pollution. Three laboratory‐scale horizontal subsurface flow constructed wetlands (CWs; planted + aeration, planted, and unplanted) were set up to investigate the feasibility of liquid digestate treatment in wetlands. Treatment capacity in different wetlands was evaluated under different influent concentrations (chemical oxygen demand [COD], 5 days biochemical oxygen demand [BOD₅], and nitrogen forms). The effect of aeration and effluent recirculation on organic matter and total nitrogen removal was investigated. Results showed that integrating intermittent aeration in CWs significantly improved the oxygen condition (p < 0.01) in the wetland bed and promoted BOD₅ removal to 90% in aerated CWs as compared with <15% in the unaerated CWs. Meanwhile, COD removal between these three wetlands did not show any difference and varied from 52 to 72% under influent concentration of 200–820 mg/L because of the high content of hard‐degradable organic matter in the liquid digestate. Intermittent aeration resulted in high ammonium removal (>98%) although the influent loading varied from 65 to 350 mg/L. However, intermittent aeration caused nitrate accumulation of 300 mg/L and limited total nitrogen (TN) removal of 33%. To intensify the TN removal, we verified effluent recirculation to increase the removal efficiency of TN to 78%. These results not only show the potential application of CWs for treatment of high‐strength liquid anaerobic digested slurry, but also indicate the significance of intermittent aeration on the enhanced removal of organic matter and ammonium.     

Seasonal and annual performance of a full-scale constructed wetland system for sewage treatment in China

A full-scale constructed wetlands system with a total area of 80 ha and treatment capability of 2.0 × 10⁴ m³ d⁻¹ was completed in October 1998 in Rongcheng, Shandong Province, China. To evaluate wastewater treatment effectiveness and seasonal performance of the system, water samples were collected and analyzed from January 1999 to December 2004. Comparison of mean inlet and outlet concentrations showed that the constructed wetland system could effectively reduce the output of SS (71.8 ± 8.4%), BOD₅ (70.4 ± 9.6%), COD (62.2 ± 10.1%), total coliform (99.7%) and fecal coliform (99.6%). However, the percent reduction of ammonia nitrogen was relatively low (40.6 ± 15.3%), and total phosphorus showed the least efficient reduction (29.6 ± 12.8%). BOD₅, COD, ammonia nitrogen, and total phosphorus removal efficiencies displayed seasonal variations. BOD₅ and COD removal was more efficient in spring and summer than in autumn and winter whereas ammonia nitrogen and total phosphorus removal was more efficient in summer and autumn than in spring and winter. Annual variation analysis shows that COD, BOD₅, and ammonia nitrogen reduction efficiencies increased from 1999 to 2004. In contrast, mean total phosphorus reduction efficiency did not change from 2001 to 2002 and began to decrease from 2003 onwards.     

A three-stage experimental constructed wetland for treatment of domestic sewage: First 2 years of operation

Hybrid constructed wetland systems have recently been used to treat wastewaters where high demand for removal of ammonia is required. However, these systems have not been used too often for small on-site treatment systems. This is because in many countries ammonia is not limited in the discharge from small systems. Hybrid systems have a great potential to reduce both ammonia and nitrate concentrations at the same time. In our study we employed a three-stage constructed wetland system consisting of saturated vertical-flow (VF) bed (2.5 m², planted with Phragmites australis), free-drained VF bed (1.5 m², planted with P. australis) and horizontal-flow (HF) bed (6 m², planted with Phalaris arundinacea) in series. All wetlands were originally filled with crushed rock (4-8 mm). However, nitrification was achieved only after the crushed rock was replaced with sand (0-4 mm) in the free-drain wetland. Also, original size of crushed rock proved to be too vulnerable to clogging and therefore, in the first wetlands the upper 40 cm was replaced by coarser fraction of crushed rock (16-32 mm) before the second year of operation started. The system was fed with mechanically pretreated municipal wastewater and the total daily flow was divided into two batches 12 h apart. The evaluation of the results from the period 2007 to 2008 indicated that such a system has a great potential for oxidation of ammonia and reduction of nitrate. The ammonia was substantially reduced in the free-drained VF bed and nitrate was effectively reduced in the final HF bed. The inflow mean NH₄-N concentration of 29.9 mg/l was reduced to 6.5 mg/l with the average removal efficiency of 78.3%. At the same time the average nitrate-N concentration rose from 0.5 to only 2.7 mg/l at the outflow. Removal of BOD₅ and COD amounted to 94.5% and 84.4%, respectively, with respective average outflow concentrations of 10 and 50 mg/l. Phosphorus was removed efficiently despite the fact that the system was not aimed at P removal and therefore no special media were used. Phosphorus removal amounted in 2008 to 65.4%, but the average outflow concentration of 1.8 mg/l is still high. The results of the present study indicate very efficient performance of the hybrid constructed wetlands, but optimal loading parameters still need to be adjusted. The capital cost of the experimental system is comparable to the conventional on-site treatment plant but the operations and maintenance costs are about one third of the conventional plant.     

Nitrogen and phosphorus removal from wastewater by subsurface wetlands planted with Iris pseudacorus

Subsurface horizontal flow constructed wetlands are being evaluated for nitrogen (N) and phosphorus (P) removal from wastewater in this study through different gravel sizes, plant densities (Iris pseudacorus), effects of retention times (1 to 10 days) on N and P removal in continuously fed gravel wetland. The inlet and outlet samples were analyzed for TKN, NH₄-N, and NO₃-N, as standard methods. The planted wetland reactor with fine (SG) and coarse (BG) gravels removed 49.4% and 31.4% TKN, respectively, while unplanted reactors removed 43.4% and 26.8% TKN. Also, the efficiencies for NH₄-N were 36.7–43% and 21.6–25.4% for SG and BG planted reactors, respectively. The efficiencies for NO₃-N were 53.5–62.5% and 21.6–25.4% for SG and BG planted reactors, respectively. Roles of plants in SG reactors for O-PO₄ were 5–12% and 3–8% in BG. Also, the roles of plants in the reactors for TP were 9% and 7.4%. The minimum effective detention time for the removal of NO₃-N was 4–5 days. The subsurface constructed wetlands planted with I. pseudacorus can be an appropriate alternative in wastewater treatment natural system in small communities.     

Life-cycle greenhouse gas emissions assessment and extended exergy accounting of a horizontal-flow constructed wetland for municipal wastewater treatment: A case study in Chile

The life-cycle greenhouse gaseous emissions and primary exergy resources consumption associated with a horizontal subsurface flow constructed wetland (HSSF) were investigated. The subject of study was a wetland for municipal wastewater treatment with a 700-person-equivalent capacity. The effects of two types of emergent aquatic macrophytes (Phragmites australis and Schoenoplectus californicus) and seasonality on greenhouse gas (GHG) gas emissions, the environmental remediation cost (ERC) and the specific environmental remediation cost (SERC) were assessed. The results indicate that GHG emissions per capita (12–22 kgCO2eq/p.e/yr) and primary exergy resources consumed (24–27 MJ/m³) for the HSSF are lower than those of a conventional wastewater treatment plant (67.9 kgCO2eq/p.e/yr and 96 MJ/m³). The SERC varied between 176 and 216 MJ/kg biological oxygen demand (BOD₅) removal, which should be further reduced by 20% for an improved BOD₅ removal efficiency above 90%. The low organic matter removal efficiency is associated with a high organic load and low bacterial development. Seasonality has a marked effect on the organic removal efficiency and the SERC, but the macrophyte species does not.     

Mountain cheese factory wastewater treatment with the use of a hybrid constructed wetland

This paper describes the activity period of an experimental hybrid wetland system placed in a cold climate region. The aim is to determine the efficiency of the system in reducing TSS, BOD₅, COD and other pollutants. The constructed wetland consists of a fat-removal unit and a basin for the storage and the distribution of the wastewater which precedes three phytoremediation beds: the first two are parallel and they work as submerged vertical flow wetland with gravel medium for an area of 180 m²; the last is a submerged horizontal flow wetland with sand medium and an area of 360 m². The CW was designed to process a total estimated BOD₅ loading rate of about 24 g m⁻² d⁻¹, which was less than half of the average actual loading rate. The wastewater treatment did not meet the required Italian law outflow limits, most likely due to BOD₅ overloading.     

Performance of integrated household constructed wetland for domestic wastewater treatment in rural areas

As environmental legislation has become stricter in recent years, the issue of wastewater treatment in rural areas has become an increasing concern. Choice of the most suitable on-site purification systems is based on the key issues of affordability and appropriateness in Chinese rural areas. This paper describes an integrated household constructed wetland (IHCW) system planted with willow (Salix babylonica) to treat household domestic wastewater in rural villages in northern China. The precast frame structure of IHCW is strong and waterproof. It can be mass-produced and installed per a standard set of specifications. The IHCW has achieved high overall removal efficiencies for BOD₅, TSS, NH₄-N, and TP: 96.0%, 97.0%, 88.4% and 87.8%, respectively. A 0.4 m biomass layer cover on the system provided significant system thermal insulation, maintaining high treatment performance in freezing winter conditions. The system is cost effective and does not need any operational energy inputs, demonstrating its feasibility for single-family use in developing countries.     

Improving water quality in polluated drains with free water surface constructed wetlands

In Egypt, disposing of partially treated or untreated domestic and industrial wastewater into agricultural drains deteriorates their water quality. A growing interest in effective low-cost treatment of polluted water and wastewater has resulted in many studies on constructed wetlands.This study evaluates free water surface constructed wetlands (by far the largest application project is named “Lake Manzala Engineered Wetland [Egypt]”) utilized to improve the water quality in Bahr El Baqar drain, which is located at the northeastern edge of the Nile Delta. This drain discharges its water into Manzala Lake, which in turn has many fishing activities and is connected to the Mediterranean Sea. The full capacity of the constructed wetland system is 25,000 m³/day. Three various flow rate wetlands were investigated; five wetland beds of high flow rate of 0.344 m³/m²-day, five wetland beds of low flow rate of 0.048 m³/m²-day and reciprocated cells of flow of 500 m³/day.The concentrations of different contaminants along the constructed wetlands system were measured to determine the treatment efficiency. The effluent was compared with the Egyptian standards of water quality in agricultural drains (Law 48/1982). Due to the high percentage of the agricultural water drain, the concentrations of contaminants in the influent were relatively low. The percentages of removal for the different contaminants were BOD₅: 52%, COD: 50%, TSS: 87%, TDS: 32%, NH₄-N: 66%, PO₄: 52%, Fe: 51%, Cu: 36%, Zn: 47% and Pb: 52%. The natural vegetation considerably increased the value of dissolved oxygen in the treated effluent. There were little differences in the removal efficiency between the high and low flow rates beds in the system.     

Long term performance of a constructed wetland for landfill leachate treatment

The constructed wetland (CW) was developed as a pilot integrated system for the capital city's old sanitary landfill site. It consisted of three interconnected beds, two of vertical flow and one of horizontal flow stage. The CW covered 311 m² with an intermittent hydraulic load of 0.5 cm d⁻¹, filled with sand media and planted with reeds and cattails. The performance efficiency of the CW systems was evaluated for 7 years through physical and chemical parameters. Some monitored parameters varied noticeable. The efficiency for COD was 50%, BOD₅ (59%), ammonia nitrogen (51%), nitrate (negative), total phosphorus (P) (53%), sulfates (negative), sulfides (49%), chlorides (35%), and Fe (84%). The average concentrations of suspended solids, COD, BOD₅, nitrate, total P, sulfates, sulfides, and Fe were below limits after treatment. The ratio between N and P showed a limited level of P for biological processes. The performance of the system did not vary significantly with regard to temperature, however, it varied with precipitation. The results showed that the CW system, as a tertiary system or as an independent system, could be a low-cost alternative for the treatment of leachate from old landfill sites.     

Hybrid Cavitation/Constructed Wetland Reactors for Treatment of Chlorinated and Non‐Chlorinated Organics

Mitigation strategies can be implemented to decrease chlorinated and non‐chlorinated organic exposures to biota of aquatic receiving systems thereby reducing associated risks. In this work, we investigated the concept of coupling a physical/chemical reactor (i.e. a cavitation reactor) with a biological reactor (i.e. a constructed wetland) in an effort to efficiently transform PCE, TCE, and petroleum in freshwater into non‐toxic chemical forms or concentrations. Rates of TCE degradation due to cavitation ranged from 0.010 to 0.026 min^‐1 with corresponding half‐lives of 69 to 27 min. Compared to controls, degradation of petroleum in water by cavitation was not detected in these experiments. After treatment in anaerobic wetland reactors, TCE and PCE decreased by more than 99 % under two flow regimes (5‐d and 20‐d HRT). In reciprocating constructed wetland reactors receiving petroleum, mean COD, BOD₅, and total Zn decreased by 90.0, 88.8, and 86.8 %, respectively, in wetland outflows compared to the initial conditions (96‐h HRT). Percent survival (96‐h) of D. magna and P. promelas increased from zero percent in initial conditions to 80.1 (± 18.9) and 80.0 (± 21.4) %, respectively, after treatment in the constructed wetland reactors. The experimental results obtained in the laboratory‐scale set‐up and the theoretical model for the hybrid reactor concept will be used to obtain the intrinsic kinetic coefficients for the appropriate reactors. This kinetic information will be used to scale‐up the hybrid reactor model concept for the same level of pollutant removal.     

Enhanced denitrification and organics removal in hybrid wetland columns: comparative experiments

This study investigated three lab-scale hybrid wetland systems with traditional (gravel) and alternative substrates (wood mulch and zeolite) for removing organic, inorganic pollutants and coliforms from a synthetic wastewater, in order to investigate the efficiency of alternative substrates, and monitor the stability of system performance. The hybrid systems were operated under controlled variations of hydraulic load (q, 0.3-0.9 m³/m² d), influent ammoniacal nitrogen (NH₄-N, 22.0-80.0 mg/L), total nitrogen (TN, 24.0-84.0 mg/L) and biodegradable organics concentration (BOD₅, 14.5-102.0 mg/L). Overall, mulch and zeolite showed promising prospect as wetland substrates, as both media enhanced the removal of nitrogen and organics. Average NH₄-N, TN and BOD₅ removal percentages were over 99%, 72% and 97%, respectively, across all three systems, indicating stable removal performances regardless of variable operating conditions. Higher Escherichia coli removal efficiencies (99.9%) were observed across the three systems, probably due to dominancy of aerobic conditions in vertical wetland columns of the hybrid systems.     

Performance evaluation of eight years experience of constructed wetland systems in Catalonia as alternative treatment for small communities

In Catalonia (Spain), a variety of different systems have been built to naturally treat liquid residues from small communities. Some of these wastewater treatment plants (WWTPs) include constructed wetlands with horizontal subsurface flow (HSSF) as secondary treatment. The present study described and characterized the performance of 11 WWTPs with secondary HSSF constructed wetland systems after an initial operating period of 8 years. The effluent concentrations of Biochemical Oxygen Demand (BOD₅), Total Suspended Solids (TSS), Total Nitrogen (TN) and Total Phosphorous (TP) were statistically analyzed, and removal efficiencies for all WWTPs including all stages in treatment were calculated. The accumulated probability functions of those parameters were evaluated to determine the influence of two different types of polishing units on the overall performance: (a) only lagoon systems and (b) lagoon systems with HSSF. The statistical analysis indicates good performance for BOD₅ and TSS. In the first case, mean concentrations below 25 mg/L were found in 9 of the 11 plants analyzed and removal efficiencies between 78 and 96% were observed. In the second case, mean concentrations below 35 mg/L were found in 8 of the 11 plants, and removal efficiencies were between 65 and 88%. For the nutrients, the removal efficiency for TN and TP were in the range of 48-66% and 39-58%, respectively. Additionally, the analysis of the influence of the polishing units did not show a significant improvement (α > 0.05) for any parameter in the wetland systems without a subsequent polishing unit. However, in the wetland systems with a polishing unit of HSSF, a significant improvement (α < 0.05) was found for the effluent's BOD₅, TN and TP concentrations but with no significant contribution in TSS management.     

Evaluation of a lab-scale tidal flow constructed wetland performance: Oxygen transfer capacity, organic matter and ammonium removal

Oxygen transfer capacity and removal of ammonium and organic matter were investigated in this study to evaluate the performance of a lab-scale tidal flow constructed wetland. Average oxygen supply under tidal operation (350 g m⁻² d⁻¹) was much higher than in conventional constructed wetlands (<100 g m⁻² d⁻¹), resulting in enhanced removal of BOD₅ and NH₄⁺. Theoretical oxygen demand from BOD₅ removal and nitrification was approximately matched by the measured oxygen supply, which indicated aerobic consumption of BOD₅ and NH₄⁺ under tidal operation. When BOD₅ removal increased from 148 g m⁻² d⁻¹ to 294 g m⁻² d⁻¹, neither exhausted oxygen from the aggregate matrix during feeding period (111 g m⁻² d⁻¹) nor effluent dissolved oxygen (DO) concentration (2.8 mg/L) changed significantly, demonstrating that the oxygen transfer potential of the treatment system had not been exceeded. However, even though DO had not been exhausted, inhibition of nitrification was observed under high BOD loading. The loss of nitrification was attributed to excessive heterotrophic biofilm growth believed to induce oxygen transfer limitations or oxygen competition in thickened biofilms.     

Integrated constructed wetland systems employing alum sludge and oyster shells as filter media for P removal

This research aimed to investigate the technical feasibility of integrated constructed wetland system consisting of a pre-filter unit and a constructed wetland (CW), in series; packed with alum sludge (AS) and oyster shells (OS) as the filter media, respectively, for nitrogen and phosphorus removal from domestic wastewater. Based on the 240 days of operation from January to August 2007, this integrated system was highly effective in removing BOD, N, P and TSS compounds which were found to be 89.5%, 68.8%, 99.4% and 89.9%, respectively. After this period, the integrated system was modified as the CW and post-filter unit, in series. The post-filter of this modified integrated system was operated during 60 days with cover for light shield and during another 60 days with no cover from September to December 2007. The treatment performance of modified integrated system was effective in removing BOD₅, N, P and TSS compounds which were found to be 91.4%, 86.8%, 99.7% and 73%, respectively, during which the post-filter had operated with no cover. To simulate high rainfall conditions, the integrated system was tested under hydraulic shock loading at the overall hydraulic retention time of 0.7 day during one day. This hydraulic shock loading conditions made BOD₅, TN, TSS concentration increase, but made no effect on P concentration. Integrated system combined a pre-filter and a CW unit or a CW unit followed by a post-filter is recommended for use in domestic wastewater which should result in high treatment performance, especially on P removal.     

A new approach in wetland systems for domestic wastewater treatment using Phragmites sp.

The conventional wetland system using Phragmites sp. is in existence since 20 years. The constructed wetland system has not gained much popularity due to large land area requirement, clogging of beds and need for capital investment. The aim of the study is to develop a new approach to the horizontal flow wetlands system (termed bio-rack) for treatment of domestic wastewater. The new system dealt with an engineered attached growth matrix and a high microbial degradation rate. The study on bio-rack was undertaken in a horizontal flow pilot plant using Phragmites sp. and experiments were carried out for various parameters. Analytical data were collected during the studies which include temperature, pH, chemical oxygen demand (COD), biological oxygen demand (BOD₅), dissolved oxygen (DO), chlorides, total dissolved solids (TDS), total suspended solids (TSS), ammonia nitrogen (NH₃-N), phosphate (PO₄-P) and most probable number (MPN). This paper also studies microbial flora present in the system by isolating and identifying the microorganisms, and also measuring total viable count (TVC). Morphological aspects with reference to the growth of plant were studied and it indicated high plant yield. At optimum hydraulic retention time (HRT) of 10 h, approximately 75.15% COD, 86.59% BOD₅, 27.54% TDS, 73.13% TSS, 8.86% Chlorides, 70.22% NH₃-N, 31.71% PO₄-P and 92.11% MPN reduction was achieved in bio-rack system. A comparative evaluation was done with the conventional wetland system and better treatment efficiency was observed in the bio-rack system. This result indicates a possibility of development of a substantially effective system compared to any other methods on constructed wetland.     

Subsurface horizontal-flow constructed wetlands for wastewater treatment: The Czech experience

The first full-scale constructed wetland (CW) in the Czech Republic was built in 1989 and there are now three tertiary systems and 50 secondary treatment facilities. We report here on the design and operational efficiencies of these facilities. All CWs have been designed with horizontal subsurface flow. Coarse sand, gravel and crushed stones with size fraction of 4–16 mm are commonly used as substrates. The area of vegetated beds ranges between 18 and 4500 m² and operational CWs are designed for population equivalent (PE) of 4 to 1,100. Common reed (Phragmites australis) is the most frequently used macrophyte species.Results from systems studied during 1994 and 1995 show that the effluent concentrations of organics and suspended solids (SS) are well below the required discharge limits. In most cases the final effluent BOD₅ concentration is <10 mg l^–1. The relationship between vegetated bed BOD₅ inflow loadings (L ₀) and outflow loadings (L) is very strong (r=0.92). Constructed wetlands with subsurface horizontal flow usually do not remove larger amounts of nitrogen and phosphorus. The results from five Czech constructed wetlands show that nitrogen removal varies among systems, but the amount of removed nitrogen is very predictable. A regression equation between nitrogen inflow loading (L ₀) and outflow loading (L) produces a strong correlation (r=0.98). The most important process responsible for phosphorus removal in wetlands is precipitation with soil Ca, Fe and Al. However, the subsurface horizontal flow constructed wetlands use mostly coarse gravel and/or sandy materials and this provides little or no P precipitation. Results from monitored systems in the Czech Republic show that the percentage phosphorus removal varies widely among systems and is lower than the percentage removal of organics and suspended solids.