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In arid countries, reclaimed water in irrigation is a widespread practice. Therefore, robust treatment designs are prerequisite to obtain effluent quality that conforms to the legal requirements for reuse and health standards. Vertical flow constructed wetlands (VFCWs) are attractive decentralized treatment plants in many countries and communities. VFCWs are capable of providing adequate treatment for organic and solids removal, even though there are limitations on nutrient and pathogen removal. Within this study, various VFCW systems were investigated, in Germany and Jordan, to optimize nitrogen removal using sustainable and low cost options to guarantee the safe reuse in Jordan.
In Germany at Langenreichenbach research facility, two-stage VFCWs planted (Phragmites australis) and unplanted were evaluated and modified to compare the role of plants over two years. Generally, there was no significant role of plants on the treatment performance. Both systems showed high removal efficiency for TOC, BOD5, and TSS over the study period. During the first year of the study, effluent TN concentrations ranged from 60 - 61 mg/L in both systems as a result of high effluent NO3--N concentrations (50 - 52 mg/L). In the second year, the systems were modified, adopting a saturated layer in the 1st stage to enhance denitrification. Average effluent TN concentrations were reduced to 45 mg/L in both systems. In addition, during modifications, the E.coli removal was enhanced in both systems achieving 4 log reduction instead of 2 log reduction during the first year of the study.
In Jordan at the Fuhais research facility, two VFCW systems were studied considering category-A (TN: 45 mg/L and NO3-N: 30 mg/L) in the Jordanian Standards for reuse in irrigation (JS 893/2006). Recirculating (ECO-1) and Multi-stage (ECO-2) VFCW designs have shown high removal efficiency of COD, TSS, and BOD5 over three years of monitoring. ECO-1 system combines simultaneous nitrification and denitrification by recycling portion of nitrified effluent into the recirculation tank. However, effluent TN and NO3--N concentrations were 55 and 44 mg/L, respectively, that the system conformed to the JS category-B (TN: 70 mg/L and NO3-N: 45 mg/L) during monitoring phase. Therefore, ECO-1 was modified by installing plastic media in the recirculation tank that attached growth increases the abundance and activity of microorganisms. TN concentration was reduced effectively of 40 mg/L, conforming to the JS category-A, whereas, NO3--N concentration was reduced to 37 mg/L, conforming to the JS category-B. However, over the study period, E.coli concentrations were conformed to the JS category-C (more than 1000 MPN/100 mL).
ECO-2 consists of two unsaturated VFCWs in series; single-pass unplanted filter followed by planted filter (Phragmites australis). E.coli removal was relatively high before modification that the effluent conformed to the JS (category-B: 1000 MPN/100 mL), achieving 4.4 log reduction. The effluent TN and NO3--N concentrations did not conform to the JS of 77 and 76 mg/L, respectively, due to insufficiency of carbon source to promote denitrification (high BOD5 removal) during monitoring phase of the study. Thus, ECO-2 was modified adopting raw wastewater step-feeding strategy that a specific volume of raw wastewater was mixed with 1st stage effluent in the mixing tank. TN and NO3--N concentrations were reduced to 52 and 50 mg/L, respectively; conforming to the JS category-B. Whereas, E.coli removal was influenced by E.coli ingress from raw step-feeding, achieving 3.5 log reduction, conforming to the JS category-C.
The short-term impact of irrigation with different water quality and quantity was investigated at the Fuhais site. Soil physicochemical and biological properties in three parallel experimental reuse plots were investigated. The plots were cultivated with lemon trees and were irrigated via a subsurface irrigation system. Moreover, each plot was divided into two subparts (A and B) whereby one part received 11 mm/day of irrigation water and the other received 6 mm/day. Using treated effluent and tap water showed the same trend of increased soli salinity (ECs). Significant difference in ECs, SAR, Mg+2, Ca+2, and Na+ were observed at 0-20 cm as a result of high evaporation and capillary rise that increased salts accumulation in the topsoil. However, using more water reduced the salts accumulation in sub soil layers due to continuous leaching. Whereas, results showed no significant variation in soil texture, structure, moisture, and infiltration rate among reuse plots and subparts. Additionally, results revealed an absence of total coliform, fecal coliform, and E.coli in the irrigated soils, indicating the effectiveness of using subsurface irrigation as a disinfection step for reuse.
Groundwater is the main source for potable water and domestic use in numerous countries around the world. However, water quality can be affected by pollution, which influences the natural environment and human health. One of the widespread pollutants in water is ammonia which is toxic to fish and causes eutrophication of lakes and wetlands. Constructed wetlands are promising in situ water treatment methods thanks to enhanced microbial growth within the plants’ rhizospheres, which creates an effective contaminant degradation zone. The ammonia in constructed wetlands can be removed either via total nitrification with further denitrification or partial nitrification coupled with anaerobic ammonium oxidation (anammox). However, so far, the role of anammox in constructed wetlands as well as its correlation with other nitrogen transformations remains unclear. The quantification of nitrogen turnover processes in constructed wetlands is difficult due to the complexity of the wetland systems.
Accordingly, the main aim of this research is to investigate pathways of ammonium removal in constructed wetlands treating contaminated groundwater. For this, several approaches were applied: a) physico-chemical parameters measurements; b) investigations of nitrogen stable isotope fractionation; c) stable isotope labelling approach; d) molecular biological methods. Also, seasonal and spatial variations in nitrogen transformations in several types of constructed wetlands (unplanted horizontal subsurface flow, planted horizontal subsurface flow, and floating plant root mat) were investigated.
The application of the stable isotope approach combined with common physico-chemical investigations enabled us to identify key factors influencing efficiency of nitrogen removal in constructed wetlands, which was the plant presence as substrate for attachment and growth of microorganisms. Ammonium removal efficiencies were not different between investigated seasons (spring, summer, and autumn), what could be explained by the fact that throughout all investigated seasons the air temperature remained high (above 10°C). While plant uptake accounted for significant part of ammonium removal during spring and summer in planted constructed wetlands, isotope fractionation patterns revealed that nitrification-denitrification were prevailing processes in planted constructed wetlands throughout the year, occurring in a linear way along the flow path, and not depending on depth in the root zone.
The research results also illustrated that in the planted horizontal subsurface flow constructed wetland, the functional genes of the nitrogen cycle were evenly distributed in a linear way along the flow path with prevalence at the superficial points. The same trend was observed for the nitrification and denitrification turnover rates using the isotope labeling techniques. Significant nitrate consumption under aerobic conditions diminishes nitrification rates and should therefore be taken into account when estimating nitrification turnover rates. This nitrate consumption was due to aerobic denitrification, the rate of which was comparable to that for anaerobic denitrification. Consequently, denitrification should not be considered as an exclusively anaerobic process. Phylogenetic analysis of hydrazine synthase (hzsA) gene clones indicated the presence of Brocadia and Kuenenia anammox species in the constructed wetland. Although anammox bacteria were detected by molecular methods, anammox activity could not be measured and hence this process appears to be of low importance in nitrogen transformations in these freshwater ecosystems.
In conclusion, this research demonstrated that combination of physico-chemical measurements with stable isotope and molecular biological approaches is an effective tool for investigation of nitrogen transforming processes in constructed wetlands. Such information is not only valuable for understanding of the processes ongoing inside these wastewater treatment facilities but also necessary for further technological improvement of constructed wetlands.