Christos S. Akratos

Total nitrogen and ammonia removal prediction in horizontal subsurface flow constructed wetlands: Use of artificial neural networks and development of a design equation

The aim of this paper is to examine if artificial neural networks (ANNs) can predict nitrogen removal in horizontal subsurface flow (HSF) constructed wetlands (CWs). ANN development was based on experimental data from five pilot-scale CW units. The proper selection of the components entering the ANN was achieved using principal component analysis (PCA), which identified the main factors affecting TN removal, i.e., porous media porosity, wastewater temperature and hydraulic residence time. Two neural networks were examined: the first included only the three factors selected from the PCA, and the second included in addition meteorological parameters (i.e., barometric pressure, rainfall, wind speed, solar radiation and humidity). The first model could predict TN removal rather satisfactorily (R(2)=0.53), and the second resulted in even better predictions (R(2)=0.69). From the application of the ANNs, a design equation was derived for TN removal prediction, resulting in predictions comparable to those of the ANNs (R(2)=0.47). For the validation of the results of the ANNs and of the design equation, available data from the literature were used and showed a rather satisfactory performance.

Water quality trends in Polyphytos reservoir, Aliakmon River, Greece

A water quality monitoring program was undertaken from June 2004 to May 2005, on a monthly basis, in Polyphytos Reservoir of Aliakmon River. Depth, water temperature, dissolved oxygen, conductivity, pH and transparency (Secchi disk) were measured in situ, while collected water samples were analyzed in the laboratory for the determination of BOD, COD, total phosphorus (TP), ortho-phosphate (OP), chlorophyll-a (Chl-a), ammonium, nitrite and nitrate, total Κjeldahl nitrogen (TKN) and heavy metals (Fe, Mn, Cu, Cr, Pb, Ni and Cd). Measured concentrations were compared to those from two previous studies conducted in July 1987 to June 1988 and January 1991 to February 1993. The following conclusions are drawn: the effect of the watershed on the lake environment, mostly through Aliakmon River, is significant, and it accelerates the eutrophication of the lake. The anoxic zones, which were defined in the lake, reinforce this conclusion. Nitrate, nitrite and ammonia were measured at lower concentrations compared to previous studies, while total phosphorus and chlorophyll-a were found at increased concentrations. The current trophic state of Polyphytos reservoir is eutrophic, based on the OECD method and Carlson’s Trophic State Indices (TSI). The concentration of BOD and COD ranged at low levels. Furthermore, the mean concentrations of metals Fe, Mn, Cu, Cr, Pb, and Cd were below the potable water standards set by WHO and EU. During most part of the study period the ratio N/P for Polyphytos reservoir was higher than 7.2:1, and therefore, phosphorus is the limiting nutrient for algal growth.

Surplus activated sludge dewatering in pilot-scale sludge drying reed beds

A pilot-scale experiment on dewatering of surplus activated sludge (SAS) is presented, where two pilot-scale vertical flow, sludge drying reed beds (SDRBs), planted with Phragmites australis are used. The bottom of the beds is filled with cobbles, connected to the atmosphere through perforated PVC ventilation tubes, in order to achieve oxygen diffusion through the overlying porous medium that is colonized by roots and an abundant nitrifying biomass. Two layers of gravel, of decreasing size from bottom to top, make the drainage layer where the reeds are planted. The two beds were fed according to the following cycle: one week feeding with SAS at rates one 30 kg/m(2)/year and the other 75 kg/m(2)/year, and resting for three weeks. The results show that planted SDRBs can effectively dewater SAS from domestic sewage, the produced residual sludge presents a high dry weight content, the degree of volume reduction depends upon the initial SAS concentration and can be of the order of 90%, and decomposition of organic matter and high levels of mineralization can be achieved. Furthermore, the percolating water is not septic. The fertilizer value of the treated SAS, which contains no added chemicals, is comparable to that of SAS treated by other methods.

Performance and modeling of a vertical flow constructed wetland–maturation pond system

A 32-month monitoring program is presented in a vertical flow constructed wetland facility, located in North Greece. The monitoring campaigns were organized every 15 days. Water quality samples were collected at the inlet, at four intermediate points (i.e., at the end of each treatment stage) and at the outlet of the system. Temperature, electrical conductivity, pH and dissolved oxygen (DO) were measured in-situ with the use of appropriate instruments at the same points of water sample collection. Water samples were analyzed for biochemical oxygen demand (BOD), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), ammonia, nitrate, nitrite, total phosphorus (TP), ortho-phosphate (OP), total suspended solids (TSS) and total coliforms (TC). Mean removal efficiencies for the monitoring period were: 90.8% for BOD, 89.0% for COD, 83.9% for TKN, 83.8% for ammonia, 38.8% for TP, 17.4% for OP, 90.4% for TSS and 99.9% for TC, indicating, for most pollutant, excellent performance of this constructed wetland (CW) system under Mediterranean climate conditions. Results showed that organic matter, TSS, TKN, ammonia, TP, OP and TC removal is not significantly affected by temperature. The collected removal data were used to produce appropriate parameter values for first-order k-C* models and develop simple models based on stepwise multiple linear regression (SMLR) analysis, in an effort to predict CW performance. These models were verified using data from another facility located in the same region. The results showed that the predictions correlate well with measured values, leading to the conclusion that the first-order models and the developed SMLR models are useful tools in the design of vertical flow (VF) CWs.

Constructed wetlands in the treatment of agro-industrial wastewater: A review

Due to their simplicity and low operation cost, constructed wetlands are becoming more prevalent in wastewater treatment all over the world. Their range of applications is no longer limited to municipal wastewater but has expanded to the treatment of heavily polluted wastewaters such as agro-industrial effluents. This paper provides a comprehensive literature review of the application of constructed wetlands in treating a variety of agro-industrial wastewaters, and discusses pollutant surface loads and the role of constructed wetland type, prior-treatment stages and plant species in pollutant removal efficiency. Results indicate that constructed wetlands can tolerate high pollutant loads and toxic substances without losing their removal ability, thus these systems are very effective bio-reactors even in hostile environments. Additionally, the review outlines issues that could improve pollutant treatment efficiency and proposes design and operation suggestions such as suitable vegetation, porous media and constructed wetland plain view. Finally, a decision tree for designing constructed wetlands treating agro-industrial wastewaters provides an initial design tool for scientists and engineers.

Modeling of Vertical Flow Constructed Wetlands

This chapter summarizes modeling efforts in vertical flow constructed wetlands (VFCWs). VFCW models include empirical (i.e., first-order k - C model, regression equation models, etc.) and mechanistic (i.e., hydrodynamic pollutant removal) models. The two most comprehensive models used in VFCW modeling, FITOVERT and Constructed Wetland 2-Dimensions model, are presented, along with a series of other models for pollutant removal prediction.

Processes and Mechanisms in Sludge Treatment Wetlands

This chapter describes the main processes and mechanisms responsible for sludge dewatering and mineralization in Sludge Treatment Wetlands. Dewatering in Sludge Treatment Wetlands is mainly achieved through water draining and evapotranspiration. Sludge mineralization is achieved through the decay of organic matter, nitrogen, phosphorus, and other organic micropollutants content and minimization of the microorganism activity (respiration activity). The various transformation/removal processes for the different pollutants are more or less the same as in Vertical Flow Constructed Wetlands with wastewater treatment, as described in part A.

Performance of Sludge Treatment Wetlands

This chapter presents the global efficiency of Sludge Treatment Wetlands from various countries, in terms of: • dewatering efficiency,

General Aspects of Sludge Management

This chapter presents general information of sludge produced during wastewater treatment. In detail, this chapter includes: (a) wastewater sludge main characteristics, (b) common treatment methods for sludge treatment, and (c) European and US legislation for sludge management.

Techno-Economic Aspects of Vertical Flow Constructed Wetlands

This chapter presents an economical and an environmental evaluation of VFCWs as a wastewater treatment technology. A discussion on the global cost of this technology takes place as also on their cost-effectiveness, environmental benefits and life cycle compared to conventional treatment technologies. Overall, VFCWs possess low investment and minimum operational costs compared to other conventional wastewater treatment methods and represent an environmental friendly technology for wastewater and sludge treatment, which places them among the sustainable treatment technologies.