Soon Keat Tan

Municipal solid waste management in China: status, problems and challenges.

This paper presents an examination of MSW generation and composition in China, providing an overview of the current state of MSW management, an analysis of existing problems in MSW collection, separation, recycling and disposal, and some suggestions for improving MSW systems in the future. In China, along with urbanization, population growth and industrialization, the quantity of municipal solid waste (MSW) generation has been increasing rapidly. The total MSW amount increased from 31.3 million tonnes in 1980 to 212 million tonnes in 2006, and the waste generation rate increased from 0.50 kg/capita/day in 1980 to 0.98 kg/capita/year in 2006. Currently, waste composition in China is dominated by a high organic and moisture content, since the concentration of kitchen waste in urban solid waste makes up the highest proportion (at approximately 60%) of the waste stream. The total amount of MSW collected and transported was 148 million tonnes in 2006, of which 91.4% was landfilled, 6.4% was incinerated and 2.2% was composted. The overall MSW treatment rate in China was approximately 62% in 2007. In 2007, there were 460 facilities, including 366 landfill sites, 17 composing plants, and 66 incineration plants. This paper also considers the challenges faced and opportunities for MSW management in China, and a number of recommendations are made aimed at improving the MSW management system.

A review on removing pharmaceutical contaminants from wastewater by constructed wetlands: Design, performance and mechanism

This paper presents a comprehensive review of the current state of research activities on the application of constructed wetlands for removing pharmaceutical contaminants from wastewater. The focus of the review was placed on the application of constructed wetlands as an alternative secondary wastewater treatment system or as a wastewater polishing treatment system. The design parameters of the reported constructed wetlands including the physical configuration, hydraulic mode, vegetation species, and targeting pharmaceuticals were summarized. The removal efficiencies of pharmaceuticals under different conditions in the wetlands were evaluated at the macroscopic level. In addition, the importance of the three main components of constructed wetlands (substrate, plants and microbes) for pharmaceutical removal was analyzed to elucidate the possible removal mechanisms involved. There is a general consensus among many researchers that constructed wetlands hold great potential of being used as an alternative secondary wastewater treatment system or as a wastewater polishing treatment system for the removal of pharmaceuticals, but relevant reported studies are scarce and are not conclusive in their findings. Current knowledge is limited on the removal efficiencies of pharmaceuticals in constructed wetlands, the removal mechanisms involved, the toxicity to constructed wetlands caused by pharmaceuticals, and the influences of certain important parameters (configuration design, hydraulic mode, temperature and seasonality, pH, oxygen and redox potential, etc.). This review promotes further research on these issues to provide more and better convincing evidences for the function and performance of larger laboratory-scale, pilot-scale or full-scale constructed wetlands.

Removal of pharmaceuticals and personal care products in aquatic plant-based systems: a review.

Pharmaceuticals and personal care products (PPCPs) in the aquatic environment are regarded as emerging contaminants and have attracted increasing concern. The use of aquatic plant-based systems such as constructed wetlands (CWs) for treatment of conventional pollutants has been well documented. However, available research studies on aquatic plant-based systems for PPCP removal are still limited. The removal of PPCPs in CWs often involves a diverse and complex set of physical, chemical and biological processes, which can be affected by the design and operational parameters selected for treatment. This review summarizes the PPCP removal performance in different aquatic plant-based systems. We also review the recent progress made towards a better understanding of the various mechanisms and pathways of PPCP attenuation during such phytoremediation. Additionally, the effect of key CW design characteristics and their interaction with the physico-chemical parameters that may influence the removal of PPCPs in functioning aquatic plant-based systems is discussed.

Application of constructed wetlands for wastewater treatment in developing countries--a review of recent developments (2000-2013).

Inadequate access to clean water and sanitation has become one of the most pervasive problems afflicting people throughout the developing world. Replication of centralized water-, energy- and cost-intensive technologies has proved ineffective in resolving the complex water-related problems resulting from rapid urbanization in the developing countries. Instead constructed wetlands (CWs) have emerged and become a viable option for wastewater treatment, and are currently being recognized as attractive alternatives to conventional wastewater treatment methods. The primary objective of this review is to present a comprehensive overview of the diverse range of practice, applications and researches of CW systems for removing various contaminants from wastewater in developing countries, placing them in the overall context of the need for low-cost and sustainable wastewater treatment systems. Emphasis of this review is placed on the treatment performance of various types of CWs including: (i) free water surface flow CW; (ii) subsurface flow CW; (iii) hybrid systems; and, (iv) floating treatment wetland. The impacts of different wetland design and pertinent operational variables (e.g., hydraulic loading rate, vegetation species, physical configurations, and seasonal variation) on contaminant removal in CW systems are also summarized and highlighted. Finally, the cost and land requirements for CW systems are critically evaluated.

Reply to comment by M. Bayani Cardenas and John L. Wilson on “Flow resistance and bed form geometry in a wide alluvial channel”

[1] The authors would like to thank Cardenas and Wilson [2006] for their valuable comments and interesting discussions. The main points raised by the commenters are summarized as follows. [2] 1. Cardenas and Wilson’s main interest lies in the estimation of the characteristic length (L00) of the separation zone behind the bed form. [3] 2. Equation (28a) of Yang et al. [2005] suggests that a may vary from 12 to 45, while Engel [1981] and Karahan and Peterson [1980] suggested that a has a value in the range of 4–6; thus it appears that equation (28a) overestimates the value of a by as much as 300% or more. The commenters concluded that the errors in a propagates into the analysis for the energy slope. [4] 3. The commenters were not clear on the values of a used for the calculation of Sc, and how Figure 4 of the Yang et al. was derived. The commenters assumed that the authors had used a value of a = 16 in the computation of energy slope S and found that the value of a was not constant. [5] The authors would like to stress that the objective of the paper is to develop a method to estimate the energy slope using easily obtainable data from the field or laboratory, such as the discharge, channel width, flow depth and sediment size. We will show that the different conclusions drawn by the commenters and the writers are attributable to the different research objectives. [6] First, the authors’ purpose is to evaluate the energy slope S which is expressed as follows (equation (6) of Yang et al.)

Pharmaceutical removal in tropical subsurface flow constructed wetlands at varying hydraulic loading rates

Determining the fate of emerging organic contaminants in an aquatic ecosystem is important for developing constructed wetlands (CWs) treatment technology. Experiments were carried out in subsurface flow CWs in Singapore to evaluate the fate and transport of eight pharmaceutical compounds. The CW system included three parallel horizontal subsurface flow CWs and three parallel unplanted beds fed continuously with synthetic wastewater at different hydraulic retention times (HRTs). The findings of the tests at 2-6 d HRTs showed that the pharmaceuticals could be categorized as (i) efficiently removed compounds with removal higher than 85% (ketoprofen and salicylic acid); (ii) moderately removed compounds with removal efficiencies between 50% and 85% (naproxen, ibuprofen and caffeine); and (iii) poorly removed compounds with efficiency rate lower than 50% (carbamazepine, diclofenac, and clofibric acid). Except for carbamazepine and salicylic acid, removal efficiencies of the selected pharmaceuticals showed significant (p<0.05) enhancement in planted beds as compared to the unplanted beds. Removal of caffeine, ketoprofen and clofibric acid were found to follow first order decay kinetics with decay constants higher in the planted beds than the unplanted beds. Correlations between pharmaceutical removal efficiencies and log K(ow) were not significant (p>0.05), implying that their removal is not well related to the compounds hydrophobicity.

Batch versus continuous feeding strategies for pharmaceutical removal by subsurface flow constructed wetland

This study evaluated the effect of continuous and batch feeding on the removal of 8 pharmaceuticals (carbamazepine, naproxen, diclofenac, ibuprofen, caffeine, salicylic acid, ketoprofen and clofibric acid) from synthetic wastewater in mesocosm-scale constructed wetlands (CWs). Both loading modes were operated at hydraulic application rates of 5.6 cm day(-1) and 2.8 cm day(-1). Except for carbamazepine, clofibric acid and naproxen, removal in CWs was significantly (p < 0.05) enhanced under the batch versus continuous mode. For all compounds tested except naproxen, values for first-order decay constants (k) for drain and fill operation were higher than that for the continuous mode of operation. Correlation between the distribution coefficient (log D(ow)) and removal efficiencies of pharmaceutical compounds in the CWs, showed that pharmaceutical removal efficiency was significantly (p < 0.1) and inversely correlated with log D(ow) value, but not with log K(ow) value.

Distribution, fractional composition and release of sediment-bound heavy metals in tropical reservoirs

The concentrations of heavy metals in bottom sediments from urban receiving waters, Kranji Reservoir and MacRitchie Reservoir in Singapore, were investigated. Distribution of the heavy metals in the bottom sediment, interstitial water and overlying water was analysed. The concentration of heavy metals in the interstitial water was found to be significantly high and exceed the water quality criteria by three to eleven times. The partitioning coefficient shows that the solubility of the metals are in the order: Mn > Zn > Cu > Pb > Fe > Al. Fractional composition of heavy metals in the sediments was determined using sequential extraction process. The results show that Cu was largely complexed by organics, 74% of Zn was in easily remobilised fractions, and 36% of Pb was in the easily reducible fraction and 47% in carbonate and ion-exchangeable fractions. Release of sediment-bound metals was studied. The results indicate that, besides pH and redox, the sediment buffering capacity is an important parameter affecting the remobilization of heavy metals from sediment.

Application of constructed wetlands for wastewater treatment in tropical and subtropical regions (2000-2013).

Constructed wetlands (CWs) have been successfully used for treating various wastewaters for decades and have been identified as a sustainable wastewater management option for developing countries. With the goal of promoting sustainable engineered systems that support human well-being but are also compatible with sustaining natural (environmental) systems, the application of CWs has become more relevant. Such application is especially significant for developing countries with tropical climates, which are very conducive to higher biological activity and productivity, resulting in higher treatment efficiencies compared to those in temperate climates. This paper therefore highlights the practice, applications, and research of treatment wetlands under tropical and subtropical conditions since 2000. In the present review, removal of biochemical oxygen demand (BOD) and total suspended solid (TSS) was shown to be very efficient and consistent across all types of treatment wetlands. Hybrid systems appeared more efficient in the removal of total suspended solid (TSS) (91.3%), chemical oxygen demand (COD) (84.3%), and nitrogen (i.e., 80.7% for ammonium (NH)4-N, 80.8% for nitrate (NO)3-N, and 75.4% for total nitrogen (TN)) as compared to other wetland systems. Vertical subsurface flow (VSSF) CWs removed TSS (84.9%), BOD (87.6%), and nitrogen (i.e., 66.2% for NH4-N, 73.3% for NO3-N, and 53.3% for TN) more efficiently than horizontal subsurface flow (HSSF) CWs, while HSSF CWs (69.8%) showed better total phosphorus (TP) removal compared to VSSF CWs (60.1%). Floating treatment wetlands (FTWs) showed comparable removal efficiencies for BOD (70.7%), NH4-N (63.6%), and TP (44.8%) to free water surface (FWS) CW systems.

Carbamazepine and naproxen: Fate in wetland mesocosms planted with Scirpus validus

Scirpus validus was grown hydroponically and exposed to the pharmaceuticals, carbamazepine and naproxen at concentrations of 0.5-2.0 mg L(-1) for an exposure duration of up to 21 d. By the end of experiment, carbamazepine elimination from the nutrient solution reached to 74%, while nearly complete removal (>98%) was observed for naproxen. Photodegradation and biodegradation played only minor roles for carbamazepine elimination, while naproxen showed a high potential for both photodegradation and biodegradation. Levels of carbamazepine ranged from 3.3 to19.0 μg g(-1) (fresh weight) in the roots and 0.3-0.7 μg g(-1) (fresh weight) in the shoots, while naproxen concentrations were 0.2-2.4 μg g(-1) (fresh weight) in the roots and 0.2-2.8 μg g(-1) (fresh weight) in the shoots. Bioaccumulation factors (BAFs) for carbamazepine ranged from 5.5 to 13.0 for roots and 0.3-1.0 for shoots, and uptake by S. validus accounted for up to 22% of the total mass loss of carbamazepine in the nutrient solutions. All BAFs for naproxen were less than 4.2 and plant uptake accounted for less than 5% of the total mass loss of naproxen, implying that plant uptake was not significant in naproxen elimination. The rather limited plant uptake of naproxen was not surprising despite the fact that its log K(ow) is close to the optimal range (1.8-3.1) for maximal potential for plant uptake. Apparently, for ionizable compounds such as naproxen, the effects of pK(a) and pH partitioning might be more important than lipophilicity.