Insights to improve the Constructed Wetlands design for the treatment of irrigated agricultural drainage waterlessons learned on the role of biota and substrate

Resumen

The arrival of excessive nutrient loading to aquatic ecosystems is provoking important environmental problems as eutrophication, with nitrate (NO3--N) from agriculture being one of the main chemical contaminants. Constructed wetlands (CWs) are created to simulate natural removal mechanisms to mitigate excess of NO3--N in water. Among all mechanisms, heterotrophic denitrification is the only mechanism that permanently removes NO3--N from water. This metabolic pathway is carried out by facultative anaerobic bacteria that reduce NO3- to N2O and N2, which are released to the atmosphere. Because heterotrophic denitrification involves the oxidation of dissolved organic carbon (DOC), DOC limitation may constrain NO3--N removal. This is the case of irrigated agricultural drainage water, which is characterised by an important C/N imbalance. Thus, providing endogenous labile DOC sources is crucial when designing CWs. In horizontal subsurface flow (HSSF) CWs, bed substrate and leaf leachates from vegetation have been described to counteract the low C/N ratio. The objective of this Thesis is to advance in the design of CWs for the treatment of irrigated agricultural drainage water and, specifically, for the NO3--N removal, considering the C/N imbalance influences the main drivers and processes involved in the NO3--N removal. To this aim, a hybrid CWs pilot plant was created with three multistage series: a HSSF CW (Phase I) with three cells as replica, a horizontal surface flow (HSF) (Phase II) and a HSSF CW (Phase III). The difference among series was the bed substrate employed in Phase I: 100 % gravel, mix of 30 % natural soil and gravel and mix of 10 % biochar and gravel. This CWs pilot plant was monitored for the first two years of functioning. In Chapter 1, the effect of adding soil or biochar to gravel was analysed on both nutrient imbalance correction and the biotic performance of CWs. Results showed the positive effect of both soil and biochar addition in plants and microorganisms growth by providing C and phosphorus (P) to the interstitial water. However, the effect of biochar was short-lived. On the contrary, the low C and P content in gravel constrained biota development. An experiment was designed in Chapter 2 to examine i) the leaf litter decomposition rates and ii) the potential microbial uptake of leaf leachates. Decomposition rates were similar among beds with only gravel or with soil addition, but it was due to different mechanisms: photodegradation predominance in gravel beds and microbial activity in soil beds. Biochar inhibited leaf litter decomposition. C leached from plants growing in soil and biochar had the greatest microbial uptake probably due to the presence of essential nutrients, contrary to gravel beds. The effects of i) different substrate types and ii) leaf leachates addition on denitrification rates were investigated in Chapter 3. Denitrification rates were found much higher in gravel+soil substrates, followed by gravel+biochar. Leaf leachates addition was positive in all substrates. Yet, leachates were crucial to allow denitrification in gravel by providing not only DOC but also nutrients that tend to limit microbial processing in this substrate type. Chapter 4 focused on the hybrid system to analyse the performance of each Phase to NO3--N removal. In Phase I, the higher contribution was found in both mixed substrates with soil (40 %) and biochar (17 %). In gravel substrate, the NO3--N removal was negligible. Phase II contributed to increase DOC availability through algae growth, but it did not have an effect on NO3--N removal in Phase III. In this Thesis, the substrate selection as a key element for the CW functioning has been demonstrated in addition to the considerable contribution of plant leachates as DOC source to enhance NO3--N removal via denitrification.