Phosphorus removal in a pilot scale free water surface constructed wetland: hydraulic retention time, seasonality and standing stock evaluation

Impact of a WWTP effluent overland flow on the properties of a mediterranean riparian soil

In this work we aim to assess the impact of a WWTP effluent overland flow on properties and nutrient concentrations of a riparian soil, in order to explore the potential of this practice as a nature-based treatment. We set two study zones of 150 m2 on the field, one control and one that received the WWTP effluent on its surface for one month. Samples were taken before and after the effluent overland flow system, to test the impact of the effluent on soil properties through a BACI design, and after 17 months, to evaluate the recovery of the soil. Two depths were studied: 0-5 cm and 5-20 cm. The effluent overland flow triggered an increase in exchangeable sodium percentage and a decrease in nitrate concentration in both depths, and an increase in ammonium concentration in 0-5 cm depth. After 17 months, there were not found relevant differences among zones. In conclusion, this practice could be used in the purpose to reduce the nutrient concentrations of WWTP effluents. This practice could be relevant for regions where WWTP effluents are discharged in low-flow or intermittent streams, such as semi-arid regions or the Mediterranean region.

Unveiling the nitrogen removal performance from microbial network establishment in vertical flow constructed wetlands

The combined effects of substrate types (natural zeolite or shale ceramsite) and hydraulic retention time (HRT, 3-day or 6-day) on nutrient removal and microbial co-occurrence networks in vertical flow constructed wetlands (VFCWs) remains to be elucidated. In this study, zeolite-packed VFCWs demonstrated superior removal rates, achieving 93.65% removal of NH4+-N and 83.84% removal of COD at 6-day HRT. The activity and establishment of microbial community were influenced by combined operating conditions. The abundances of Amx, amoA, nxrA, and nosZ genes increased with longer HRTs in zeolite-packed VFCWs. Additionally, a 6-day HRT significantly increased the relative abundances of Proteobacteria and Nitrospirae. At the species level, zeolite-packed VFCWs exhibited ecological niche sharing as a coping strategy in response to environment changes, while ceramsite-packed VFCWs displayed ecological niche differentiation. Both zeolite-packed and ceramsite-packed VFCWs established functional networks of nitrogen-transforming genera that utilized ecological niche differentiation strategies.

Removal effect of typical pollutants from stormwater runoff in ecological ditches

Ecological ditches are a typical ecological facility for controlling road stormwater runoff pollution; they mainly remove harmful pollutants from runoff through plant absorption, retention and sedimentation, ecological adsorption, and microbial action. In this paper, according to the transport form of rainwater in the ditches, the removal effects of two different types of ditches on nitrogen, phosphorus, heavy metals, and other pollutants were simulated under three conditions of rainfall, slow flow, and still water, respectively, and their operating characteristics were analyzed. The results showed that the removal rate of TN in the two ecological ditches under slow flow conditions showed a downward trend as a whole with the increase of hydraulic load, and the suitable hydraulic load for TN removal should be selected as 0.3 m3/(m2 day). Under the simulated rainfall conditions, the TN removal rates of no. 1 and no. 2 ditches were 26.1-37.2% and 24.9 ~ 52.5%, respectively, and the TP removal rates were 44.6 ~ 63.3% and 36.1 ~ 62.1%. After 19.4 h and 22.1 h in the static state, the TP concentration in no. 1 ditch and no. 2 ditch reached the surface V water standard, and the average removal rate of TP was 74.7% and 53.7%, respectively. This paper provides a reference for selecting suitable parameters and optimizing the operational performance of ecological ditches to reduce runoff pollutants more effectively.

The efficiency of full-scale subsurface constructed wetlands with high hydraulic loading rates in removing pharmaceutical and personal care products from secondary effluent

Constructed wetlands (CWs) are usually operated at low hydraulic load rates (HLRs) of < 0.5 m³/m²/d, and can efficiently remove pharmaceuticals and personal care products (PPCPs) from wastewaters. They however often occupy a large area of land, especially when treating the secondary effluent from wastewater treatment plants (WWTPs) in megacities. High-load CWs (HCWs) with an HLR ≥ 1 m³/m²/d, requiring smaller land areas, are a good option for urban areas. However, their performance for PPCP removal is not clear. In this study, we evaluated the performance of three full-scale HCWs (HLR: 1.0-1.3 m³/m²/d) to remove 60 PPCPs, and found they had a stable removal performance and a higher areal removal capacity than the previously reported CWs operated at low HLRs. We verified the advantages of HCWs by testing the efficiency of two identical CWs at a low HLR (0.15 m³/m²/d) and a high HLR (1.3 m³/m²/d) fed with the same secondary effluent. The areal removal capacity during the high-HLR operation was 6-9 times higher than that during the low-HLR operation. A high dissolved oxygen content, and low COD and NH₄-N concentrations in the secondary effluent were critical for the robust PPCP removal by tertiary treatment HCWs.

Response of Typha domingensis Pers. in floating wetlands systems for the treatment of water polluted with phosphorus and nitrogen

The aims of this work were to evaluate the effects of P and N on the tolerance and root morphometry of Typha domingensis plants, and their implication in removal efficiency in floating treatment wetlands (FTWs). Pilot-scale plastic reactors containing plants, sediment, and tap water were arranged. FTWs consist of a plastic net, and buoyancy was provided by a PVC frame. After plant acclimation, 38 L of the synthetic effluent containing 10 mg L^-1 N + 2 mg L^-1 P was added to the reactors as follows: reactor A (with FTWs), reactor B (without FTWs), reactor BC (biological controls), and reactor CC (chemical control). Reactors were arranged in triplicate. During the experiment, three effluent dumps were made. The removals of SRP and TP were significantly higher in reactor A than in reactor B. N-NH₄⁺ removal was not significantly different between reactors A and B, while N-NO₃^- removal from water was higher in reactor A than in reactor B. At the end of the experiment, chlorophyll concentration and aerial and submerged (roots and rhizomes) biomass increased significantly in reactor A. TP concentrations were not different between rhizomes and leaves, while the lowest concentrations were observed in roots. The TKN in tissues was significantly higher in roots and rhizomes than in aerial parts. In plants exposed to the experimental solution, the internal and external root morphology changed. The use of FTWs is a promising strategy for the sustainable treatment of nutrient polluted water bodies.

Low biological phosphorus removal from effluents treated by slow sand filters

The legislation for environment protection requires strict controls of the wastewater releasing in water bodies. The wastewater treatment plants (WWTP) have been used for organic matter degradation; however, the residual total phosphorus (TP) removal has not been efficient. TP and nitrogen present in wastewater are associated to eutrophication of water bodies and algae growth. Therefore, this study discusses the efficiency of phosphorus removal by a slow filter (SF), complementary to a WWTP and the microbial community involved. The results showed that the use of SF, with or without macrophytes, is not suitable to remove TP. Spatial variation in microbial communities distributed in three distinct zones was identified in the SF. Proteobacteria, Bacteroidetes, Chloroflexi and Firmicutes covered the hydrolytic and fermentative bacteria. The acetogenesis, nitrification, and denitrification, as well as the removal of phosphorus from the effluent, were performed by representatives affiliated to different groups. Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria among these, Dokdonella sp., Frateuria sp., Comamonas sp., Diaphorobacter sp., Nitrosospira sp., Ferruginibacter sp., Flavobacterium sp., and the uncultured OD1 were the most abundant bacteria in the SF. The low efficiency for TP removing from SF effluents can be explained by the low abundance of phosphorus accumulating organisms (PAOs), with the association of the low concentration of biodegradable organic matter in the inlet effluent. Therefore, the alternative to using SF as a complement to WWTPs, as recommended by some Brazilian environmental agencies, did not prove to be viable and new approaches must be evaluated. KEY POINTS: • The phosphorus removal was performed by a slow filter system in a WWTP but obtained a low efficiency. • Microbial spatial variation was distributed into distinct zones from slow filter. • Low abundance of PAOs was observed due to the low availability of organic matter.

Enhanced nitrogen and phosphorus removal by natural pyrite-based constructed wetland with intermittent aeration

Four subsurface flow constructed wetlands (SFCWs) filled with different substrates including ceramsite, ceramsite+pyrite, ceramsite+ferrous sulfide, and ceramsite+pyrite+ferrous sulfide (labeled as SFCW-S1, SFCW-S2, SFCW-S3, and SFCW-S4) were constructed, and the removal of nitrogen and phosphorus by these SFCWs coupled with intermittent aeration in the front section was discussed. The key findings from different substrate analyses, including nitrification and denitrification rate, enzyme activity, microbial community structure, and the X-ray diffraction, revealed the nitrogen and phosphorus removal mechanism. The results showed that the nitrogen and phosphorus removal efficiency for SFCW-S1 always remained the lowest, and the phosphorus removal efficiency for SFCW-S4 was recorded as the highest one. However, after controlling the dissolved oxygen by intermittent aeration in the front section of SFCWs, the nitrogen and phosphorus removal efficiencies of SFCWs-S2 and S4 became higher than those of SFCW-S1, and SFCW-S3. It was noticed that the pollutants were removed mainly in the front section of the SFCWs. Both precipitation and adsorption on the substrate were the main mechanisms for phosphorus removal. A minute difference of nitrification rate and ammonia monooxygenase activity was observed in the SFCWs' aeration zone. The denitrification rates, nitrate reductase, nitrite reductase, and electron transport system activity for SFCW-S2 and SFCW-S4 were higher than those detected for SFCW-S1 and SFCW-S3 in the non-aerated zone. Proteobacteria was the largest phyla found in the SFCWs. Moreover, Thiobacillus occupied a large proportion found in SFCW-S2, and SFCW-S4, and it played a crucial role in pyrite-driven autotrophic denitrification.