Globally, alum sludge is an easily, locally and largely available by-product from water treatment plants where aluminium sulphate is used as the coagulant for raw water purification. Owing to the high content of Al ions (29.7 ± 13.3% dry weight) in alum sludge and the strong affinity of Al ions to adsorb various pollutants especially phosphorus (P), alum sludge (in the form of dewatered cakes) has been investigated in recent years as a low-cost alternative substrate in constructed wetland (CW) systems to enhance the treatment efficiency especially for high strength P-containing wastewater. Long-term trials in different scales have demonstrated that the alum sludge-based CW is a promising technique with a two-pronged feature of using ‘waste’ for wastewater treatment. Alum sludge cakes in CW can serve as a medium for wetland plant growth, as a carrier for biofilm development and as a porous material for wastewater infiltration. After the intensive studies of the alum sludge-based CW system, this paper aims to address the key issues and concerns pertaining to this kind of CW system. These include: (1) Is alum sludge suitable for reuse in CWs? (2) Is Al released from the sludge a concern? (3) What is the lifespan of the alum sludge in CWs? (4) How can P be recovered from the used alum sludge? (5) Does clogging happen in alum sludge-based CW systems and what is the solution?
The removal of phosphorus (P) is one of the major targets in domestic and municipal wastewater treatment while the important process in P removal is adsorption. Constructed wetlands (CWs) play an increasing role in various wastewater treatments while many attempts have been made to seek novel substrates in CWs, other than the conventional substrates of soil, sand and gravel, to achieve enhanced P removal/immobilization (Ballantine & Tanner 2010). One such effort is the use of dewatered alum sludge cakes in CWs to develop the alum sludge-based CW system (Zhao et al. 2008). This development is derived from the fact that: (1) alum sludge is, globally, an easily, locally and largely available by-product from water treatment plants where aluminium sulphate is used as the coagulant for raw water purification (Babatunde & Zhao 2007); and (2) Al ions are the major component in alum sludge (up to 29.7 ± 13.3% dry weight) and it has strong affinity to, and adsorbs, various pollutants, especially P (Huang & Chiswell 2000; Kim et al. 2002; Ippolito et al. 2003; Makris et al. 2005; DeWolfe 2006; Yang et al. 2006; Razali et al. 2007; Babatunde et al. 2008; Li et al. 2013).
The first laboratory scale alum sludge-based CW system was set up in University College Dublin in 2006 and long-term trials in different scales have been conducted since then. In 2014, the first group of five large scale applications of alum sludge-based CW systems has been implemented in China for domestic wastewater treatment in newly established residence areas in Shaanxi Province (Zhao et al. 2014). Ample trials have demonstrated that the alum sludge-based CW is a promising technique with a two-pronged feature of using ‘waste’ for wastewater treatment.
Based on the intensive studies of the alum sludge-based CW system in different scales, this paper aims to address the key issues and concerns pertaining to this kind of CW system. These include: (1) Is alum sludge suitable for reuse in CWs? (2) Is Al released from the sludge a concern? (3) What is the lifetime of the alum sludge in CWs? (4) How can P be recovered from the used alum sludge? (5) Does clogging happen in alum sludge-based CW systems and what is the solution?
MAKING ALUM SLUDGE AN ENVIRONMENTALLY SAFE MATERIAL FOR REUSE
Is alum sludge a suitable material for use in CW
Since alum sludge is derived from the residual of treatment of raw water which contains mainly turbidity, colour, suspended clays and humic substances, depending on the raw water, it is unlikely to contain a substantial quantity of toxic substances. The alum sludge has been tested for the purposes of determining whether it can be: (1) a carrier for biofilm development; (2) a medium for wetland plant growth; and (3) an adsorbent for P adsorption to assess its suitability as a substrate in CWs and compared with other materials traditionally used in CWs. Details of these trials in our group can be referred to Babatunde et al. (2009). Among the numerous characteristics related to the above three purposes, bulk density, specific surface area (SSA) and the particle size distribution (PSD) of the alum sludge were examined. The values of d10 and d60 and the uniformity coefficient (UC) were determined. Surface morphology and microstructure were examined using scanning electron microscopy (SEM) to visualize inner porosity, surface properties and potential environment for biofilm bacteria. The SEM was further combined with energy dispersive X-ray (EDX) to determine the composition and relative distribution of elements particularly on the alum sludge surface. X-ray diffraction of randomly oriented powders of the samples was carried out while the FTIR spectra were measured. The chemical properties of pH, loss of ignition, electrical conductivity (EC), the elemental metal composition via ICP-MS and the specific anions (Cl-, , ) and humic acid content (expressed as total organic carbon (TOC)) were all determined.
The characterization results show that the alum sludge had a bulk density of 1.18 ± 0.11 g/cm3 and a porosity of 45%, both of which are comparable to values of 0.7–1.83 g/cm3 (bulk density) and 30–54.4% (porosity) reported in the literature for potential candidate materials for CWs (Drizo et al. 1999; Roger 2000; Del Bubba et al. 2003). From the PSD, the values of d10 and d60 and UC were computed to be 0.5, 1.8 and 3.6, respectively, and these are consistent with several guidelines for securing adequate hydraulic conductivity and minimizing the risk of clogging (IWA 2000). The microstructure of the alum sludge suggests a ‘rough’ surface that would be ideal for biofilm growth. The SSA ranged from 28.0 to 41.4 m2/g. In comparison to the range of SSA of other candidate wetland materials reported in the literature such as 2.6–3.9 m2/g (Roger 2000) and 6.8–31.4 m2/g (Drizo et al. 1999), the alum sludge used herein can be seen to have a comparatively higher SSA. This further illustrates that alum sludge has an adequate surface area for biofilm growth and attachment. The micropore, mesopore and pore volumes of the alum sludge were determined to be 0.0125 cm3/g, 0.0210 cm3/g and 0.0410 cm3/g, respectively. The mesopore volume was about 1.7 times the micropore volume and the mesopores also accounted for about 51.2% of the entire pore volume of the alum sludge. The EC of the alum sludge ranged from 0.104 to 0.140 dS/m, and thus the sludge can be considered non-saline (Smith & Doran 1996). The EC is also well below the 4 dS/m associated with reduced plant growth due to salinity (Dayton & Basta 2001). It is important to note that microbial mediated processes, which are the basis of wetland operations, are very sensitive to soil EC. The elemental composition of the alum sludge trial in Ireland and in China, in comparison with an Irish top soil, is given in Table 1. The Al component in the alum sludge is about 10–40 times greater than the soil analyzed. The results demonstrated that alum sludge has an ideal surface for biofilm growth. Both the pH (6.8–7.4) and the EC of the sludge showed that it should suitably support plant growth (Figure 1) while chemical analysis also indicate that there is no component of the sludge that should significantly preclude its use as a substrate in CWs.
|Element (mg/g)||Irish alum sludge||Chinese alum sludge||Irish top soil|
|Element (mg/g)||Irish alum sludge||Chinese alum sludge||Irish top soil|
Is Al released from the alum sludge a concern
One of the concerns of the beneficial reuse of alum sludge in CW is the possible Al release from the sludge since the sludge is enriched in Al. Accordingly, a brief review of the relevant literature regarding Al release during alum sludge reuse was conducted and Al monitoring from a pilot-scale four-stage CW study conducted in the field was carried out (Babatunde et al. 2011). Actually, several regulations have been promulgated in relation to Al concentration in drinking waters and effluents for discharge. In Ireland and UK, the prescribed limit for Al discharge into all waters is 0.2 mg/L (Council Directive 98/83/EC 1998). For drinking waters, the World Health Organization suggests a maximum limit of 0.2 mg/L (World Health Organization 1996) while in the USA, the United States Environmental Protection Agency secondary drinking water regulation stipulates a range of 0.05–0.2 mg/L (Kvech & Edwards 2002).
The results obtained in the four-stage pilot-scale study showed that the level of Al leached was quite low ranging from 0.02 to 0.06 mg/L. The overall concentration of Al in the effluents was well below the regulatory guideline limit of 0.2 mg/L except during the first three weeks of the total 42 weeks monitoring. It has been found that in the first three weeks leached levels of Al increased across the stages from the first stage to the fourth stage. The highest Al concentration of the final effluent (fourth stage) was 0.7 mg/L. Even though the levels of Al found in the effluents do not represent an imminent environmental or health risk after the first three weeks, periodic determinations are advisable. On the effect of P adsorption on Al release, analysis of P removed and Al released in each stage of the CW revealed an inverse trend between P adsorbed and Al released across the four stages of the CW. It therefore follows that P adsorption onto the alum sludge may contribute to reducing Al leaching from the alum sludge, but it is not a direct relationship. Mortula et al. (2007) reported similar findings in their studies that P adsorption on alum sludge may have an insignificant effect on Al release.
What is the lifespan of the alum sludge in CWs
The lifespan of the alum sludge employed in CWs is another concern. Here, the lifespan refers to two aspects of the use of alum sludge as a low-cost adsorbent and substrate in CW systems. One is the effect of ageing of alum sludge on its P adsorption ability and the other is the lifespan when the alum sludge used in CWs becomes fully saturated and thus loses its ability for further adsorption of P in wastewater. These two aspects have been well investigated in previous studies (Yang et al. 2008; Zhao et al. 2009). By examining the physical–chemical characteristics and the P-adsorption capacity of alum sludge aged 6, 12 and 18 months, Yang et al. (2008) reported that the aged alum sludge (up to 18 months) did not lose its P adsorption ability compared with the freshly produced alum sludge in water treatment plants. These results are encouraging regarding the storage of alum sludge for future reuse. In a long-term CW system trial, Zhao et al. (2009) reported the estimation of the lifespan of the alum sludge and claimed that, regarding its saturation with P, the alum sludge can be used in CWs for 4–17 years depending on the P concentration of the wastewater: four years refers to farmyard wastewater with high P concentration (ranged 28 ± 15 mg-P/L) while 17 years is for normal domestic wastewater (up to 6–8 mg-P/L). It is noted that the CW system is a multi-stage system. When the first stage becomes saturated the remaining stages function well. Interestingly, Li et al. (2013) characterized five alum sludges collected from different regions across China and reported that one of the sludges' lifespan can reach 302 years under the test of sludge particle size between 0.25 and 0.6 mm. Of course, this can be used as a guide, but it is yet to be fully verified on a field scale. Actually, in practical application, with the increase of particle sizes, P-adsorption capacity will be reduced. As a reference, the suggested timespan of wetland substrate (soil, sand and gravel) is normally 15 years for the purpose of CW design (IWA 2000).
How can P be recovered from the used alum sludge in CW
It has been demonstrated that over 90% of P can be adsorbed/removed by the alum sludge-based CW system (Zhao et al. 2009). However, there is concern as regards the final fate of the P-saturated sludge after reuse in CWs. Therefore, a special investigation was conducted to explore the feasibility and effectiveness of P recovery from saturated alum sludge in CWs. A P-recovery process was developed (Zhao et al. 2011, 2013), which involved: (1) P-desorption by H2SO4; (2) the Fenton reagent reaction for decolouration of the red-brown sulphuric acid leachate (RSAL); followed by (3) precipitations of aluminium phosphate (AlPO4) (termed as AlP-1 and AlP-2), calcium phosphate (termed as CaP-1 and CaP-2), and further precipitations of aluminium compounds of either Al(OH)3 (alumina trihydrate) or Alq3 by pH adjustment. Alq3 is an organic electronic material and could be used as an efficient electroluminescent material. The strategic roadmap of the P and Al recovery is illustrated in Figure 2 while the identification of the precipitations is shown in Figure 3.
It has been generally agreed that P is a non-replaced resource with limited sources in nature. Therefore, a wiser wastewater treatment would be to consider resource recovery, especially P since it is one of the vital components of DNA, the key element of adenosine triphosphate (ATP), and the energy supplier for physiological processes. Although the purity, structure, characteristics and production control of the recovered compounds via used alum sludge in CWs for P-rich wastewater treatment are worthy of further investigation, this study successfully explored the technical methodology for the transformation of P recovery. However, it is obvious that the processes are complicated and the cost should be high. Therefore, it is fair to say that the P recovery after the slum sludge reuse in CW system is feasible from a technical point of view, but infeasible from economic point of view (at least from a foreseeable period). An alternative option is to treat the used alum sludge as P-enriched fertilizer for forest and garden use.
Does clogging happen in alum sludge-based CW systems and what is the solution
Substrate clogging is the most serious problem in CW practice. Like conventional CW systems, the alum sludge-based CW has encountered the substrate clogging problem after 14 months of operation especially in the first stage (vertical subsurface flow bed) of the system in a pilot-scale trial in Ireland as obvious ponding was observed (Figure 4(a)). The reason could be associated with the high strength animal farm wastewater and high operational hydraulic loading in the trial. Although the clogging has been well recognized in CW research and development, there is no effective solution to relieve it. Efforts have been made in the trial to seek a solution. These include first stage resting, earthworms addition (Figure 4(b)), and the replacement of the first stage with an anti-sized gravel bed (Figure 4(c)) (Zhao et al. 2004). It has been demonstrated that the bed resting and anti-sized gravel bed are useful approaches for clogging relief and thus they are suggested in practice in the CW application.
In terms of wastewater treatment in rural areas and isolated industrial estates, CWs are one of the promising technologies that can provide cost-effective wastewater treatment alongside sustainable development. In particular, increasing demands for decentralized systems for sewage treatment in rural areas and industrial settings offer great opportunity for wide application of CWs. It is well known that the substrate in CWs plays a key role in enhancing the treatment efficiency. A special novelty of the developed alum sludge-based CW system is the use of a hitherto landfill designated by-product, alum sludge, as the substrate in the CW as opposed to the traditional media of gravel, sand and local soils. The series of trials described in this paper is for the purpose of scaling up such preliminary investigations to field scale level and towards full-scale large application. The use of alum sludge as a substrate in CW is justifiable, both economically and environmentally, as it is an inevitable by-product largely produced in water treatment plants worldwide and it is locally and easily available free of charge. Accordingly, the reuse of alum sludge will bring about considerable saving in landfill costs and the need for landfill capacity. Together with the high P removal, the most recent study has further demonstrated that the alum sludge-based CW can achieve high nitrogen removal with proper operational strategies being adopted (Hu et al. 2014), indicating the high reality of the system for nutrient removal, which is the real target for wastewater treatment in modern society.
Currently, five full-scale applications of the alum sludge-based CW system for domestic wastewater treatment in five newly established villages in Northwest China, Shaanxi Province have been completed (Zhao et al. 2014) and more field data will be presented accordingly in the near future. However, it should be pointed out that even though the alum sludge-based CW system has demonstrated obvious advantages over conventional CWs, there are constraints that should be carefully considered for large-scale applications. These include logistics of the use of the alum sludge at field scale vis-à-vis the application mode of the alum sludge, and the typical P adsorption capacity of the alum sludge in the field. Being an unconventional substrate material for CW systems, the application mode of the alum sludge (i.e. either freshly dewatered alum sludge; aged dewatered alum sludge; dried alum sludge; granulated alum sludge) should be carefully considered as this will influence the transport and handling logistics. Furthermore, adequate attention should be paid to the P-loading capacity of the alum sludge used in a full-scale CW as this would differ from computations based on small-scale work. In addition, the CW will likely require regular aluminium monitoring as the effects of aluminium level beyond the recommended safe limit could be lethal. This may incur additional cost for the CW operation. More significantly, clogging is an inevitable long-term operational drawback of CW systems, and it is important to be aware of this as a possible operational challenge in the long-term basis.
Since the alum sludge is treated as a ‘waste’ for landfilling, the reuse of the sludge in CWs to achieve better wastewater treatment efficiencies, especially for enhanced P immobilization, represents the novelty of the alum sludge-based CW system. However, attention should be paid to the key issues of the possible large application of such CW systems. Although intensive trials have demonstrated that alum sludge can serve as a medium for wetland plant growth, as a carrier for biofilm development and as a porous material for wastewater infiltration, much attention should be paid to the lifespan of the alum sludge and the potential clogging of the bed when the alum sludge-based CW system is proposed. Al release from the sludge does not seem to present a problem while P recovery strategies from the saturated alum sludge have been well developed in spite of the high cost of recovery. Overall, an alum sludge-based CW system is a promising technique and is highly possible for large-scale application.
The authors would like to acknowledge the research funding provided by the Irish state Environmental Protection Agency through the Environmental Technologies Scheme (project no. 2005-ET-S-7-M3, 2005-ET-MS-38-M3) and the Irish state Department of Agriculture, Fisheries and Food under the Research Stimulus Fund (project no. RSF 07-528). This study is also financially supported by the China Central University Fund (2013G1502043 & 2013G1291073) and the Key Programme of National Natural Science Foundation of China (no. 41230314). The Environmental Protection Agency, Feng County, Shaanxi Province, P.R. China are thanked for the full scale CW study.