Esther Llorens

Sludge treatment wetlands: a review on the state of the art

Sludge management has become a key issue in wastewater treatment, representing some 20-60% of the operational costs of conventional wastewater treatment plants. The high water content of the sludge results in large daily flow rates to be handled and treated. Thus, the search for methods to improve sludge volume reduction continues to be of major interest. The technology known as sludge treatment wetlands has been used for sludge dewatering since the late 1980s. Major advantages include its low energy requirements, reduced operating and maintenance costs, and a reasonable integration in the environment. However, the number of plants in operation is still low in comparison with conventional technologies. This study represents a review of the state of the art of sludge treatment wetlands. The main characteristics and operational aspects of the technology are described, including a summary of the main results reported in the literature. Finally, the efficiency of sludge treatment wetlands versus conventional treatments is compared.

Water quality improvement in a full-scale tertiary constructed wetland: effects on conventional and specific organic contaminants.

The surface flow constructed wetland (SF CW) in Can Cabanyes (Granollers, Catalonia, northeastern Spain) was created as a part of a series of activities aimed at restoring a highly impacted fluvial peri-urban zone. The system is fed with a small part of the secondary effluent, which is not completely nitrified, from an urban wastewater treatment plant. Effluents in the SF CW were sampled between 2003 and 2006 for physical and chemical parameters and faecal bacteria indicators. In addition, 8 pharmaceuticals and personal care products (PPCPs) were measured in June 2005 and February 2006. The system showed a good reliability for ammonium and faecal bacteria removal, with average ammonium efficiencies between 64 and 87% and a removal of approximately 2 logarithmic units of Faecal Coliforms. A clear seasonal trend was observed for ammonium. The results for PPCPs demonstrated that the wetland has a good capacity for removing a large variety of these compounds; the removal efficiencies were higher than 70% for most of them, with the exception of clofibric acid (34%) and carbamazepine (39%). Although the chemical oxygen demand and total suspended solid removal rates were either low or zero because of the permanent eutrophic conditions of the system and the low incoming concentrations, the effluent quality is generally considered to be quite good.

Sludge dewatering and stabilization in drying reed beds: Characterization of three full-scale systems in Catalonia, Spain

Optimization of sludge management can help reducing sludge handling costs in wastewater treatment plants. Sludge drying reed beds appear as a new and alternative technology which has low energy requirements, reduced operating and maintenance costs, and causes little environmental impact. The objective of this work was to evaluate the efficiency of three full-scale drying reed beds in terms of sludge dewatering, stabilization and hygienisation. Samples of influent sludge and sludge accumulated in the reed beds were analysed for pH, Electrical Conductivity, Total Solids (TS), Volatile Solids (VS), Chemical Oxygen Demand, Biochemical Oxygen Demand, nutrients (Total Kjeldahl Nitrogen (TKN) and Total Phosphorus (TP)), heavy metals and faecal bacteria indicators (Escherichiacoli and Salmonella spp.). Lixiviate samples were also collected. There was a systematic increase in the TS concentration from 1-3% in the influent to 20-30% in the beds, which fits in the range obtained with conventional dewatering technologies. Progressive organic matter removal and sludge stabilization in the beds was also observed (VS concentration decreased from 52-67% TS in the influent to 31-49% TS in the beds). Concentration of nutrients of the sludge accumulated in the beds was quite low (TKN 2-7% TS and TP 0.04-0.7% TS), and heavy metals remained below law threshold concentrations. Salmonella spp. was not detected in any of the samples, while E. coli concentration was generally lower than 460MPN/g in the sludge accumulated in the beds. The studied systems demonstrated a good efficiency for sludge dewatering and stabilization in the context of small remote wastewater treatment plants.

Bacterial transformation and biodegradation processes simulation in horizontal subsurface flow constructed wetlands using CWM1-RETRASO

The performance and reliability of the CWM1-RETRASO model for simulating processes in horizontal subsurface flow constructed wetlands (HSSF CWs) and the relative contribution of different microbial reactions to organic matter (COD) removal in a HSSF CW treating urban wastewater were evaluated. Various different approaches with diverse influent configurations were simulated. According to the simulations, anaerobic processes were more widespread in the simulated wetland and contributed to a higher COD removal rate [72-79%] than anoxic [0-1%] and aerobic reactions [20-27%] did. In all the cases tested, the reaction that most contributed to COD removal was methanogenesis [58-73%]. All results provided by the model were in consonance with literature and experimental field observations, suggesting a good performance and reliability of CWM1-RETRASO. According to the good simulation predictions, CWM1-RETRASO is the first mechanistic model able to successfully simulate the processes described by the CWM1 model in HSSF CWs.

Reactive transport simulation in a tropical horizontal subsurface flow constructed wetland treating domestic wastewater.

A promising approach to the simulation of flow and conversions in the complex environment of horizontal subsurface flow constructed wetlands (HSSF-CWs) is the use of reactive transport models, in which the transport equation is solved together with microbial growth and mass-balance equations for substrate transformation and degradation. In this study, a tropical pilot scale HSSF-CW is simulated in the recently developed CWM1-RETRASO mechanistic model. The model predicts organic matter, nitrogen and sulfur effluent concentrations and their reaction rates within the HSSF-CW. Simulations demonstrated that these reactions took place simultaneously in the same (fermentation, methanogenesis and sulfate reduction) or at different (aerobic, anoxic and anaerobic) locations. Anaerobic reactions occurred over large areas of the simulated HSSF-CW and contributed (on average) to the majority (68%) of the COD removal, compared to aerobic (38%) and anoxic (1%) reactions. To understand the effort and compare computing resources needed for the application of a mechanistic model, the CWM1-RETRASO simulation is compared to a process-based, semi-mechanistic model, run with the same data. CWM1-RETRASO demonstrated the interaction of components within the wetland in a better way, i.e. concentrations of microbial functional groups, their competition for substrates and the formation of intermediary products within the wetland. The CWM1-RETRASO model is thus suitable for simulations aimed at a better understanding of the CW system transformation and degradation processes. However, the model does not support biofilm-based modeling, and it is expensive in computing and time resources required to perform the simulations.

Modelling of arsenic retention in constructed wetlands

A new model was developed in order to simulate the most significant arsenic retention processes that take place in constructed wetlands (CWs) treating high arsenic waters. The present contribution presents the implementation phases related to plants (arsenic uptake and accumulation, root arsenic adsorption, and root oxygen release), showing the first simulation results of the complete model. Different approaches with diverse influent configurations were simulated. In terms of total arsenic concentrations in effluent, the simulated data closely matched the data measured in all evaluated cases. The iron and arsenic species relationships, and the arsenic retention percentages obtained from simulations, were in agreement with the experimental data and literature. The arsenic retention efficiency increased whenever a new phase was implemented, reaching a maximum efficiency range of 85-95%. According to the quality of the obtained results, it can be considered that the implementation of all steps of RCB-ARSENIC provided reasonably good response values.

Properties of biosolids from sludge treatment wetlands for land application

Sludge treatment wetlands consist of constructed wetlands which have been upgraded for sludge treatment over the last decades. Sludge dewatering and stabilisation are the main features of this technology, leading to a final product which may be recycled as an organic fertiliser or soil conditioner. In this study, biosolids from full-scale treatment wetlands were characterised in order to evaluate the quality of the final product for land application, even without further post-treatment such as composting. Samples of influent and treated sludge were analysed for pH, Electrical Conductivity, Total Solids (TS), Volatile Solids (VS), Chemical Oxygen Demand (COD), Dynamic Respiration Index (DRI), nutrients (Total Kjeldahl Nitrogen (TKN), Total Phosphorus (TP) and Potasium (K)), heavy metals and faecal bacteria indicators (E. coli and Salmonella spp.). According to the results, sludge water content and therefore sludge volume are reduced by 25%. Organic matter biodegradation leads to VS around 43–44%TS and COD around 500 g kgTS−1. The values of DRI24 h (1000–1500 mgO2 kgTS−1 h−1) indicate that treated sludge is almost stabilised final product. Besides, the concentration of nutrients is quite low (TKN~4%TS, TP~0.3%TS and K~0.2–0.6%TS). Both heavy metals and faecal bacteria indicators meet current legal limits for land application of the sludge. Our results suggest that biosolids from the studied treatment wetlands could be valorised in agriculture, especially as soil conditioners.