Urban Institute Ireland Research Collection

The concept, design and performance analysis of a four-stage novel constructed wetland system (CWs) capable of enhanced and simultaneous removal of phosphorus (P) and organic matter (OM) from wastewaters is described. Alum sludge, a largely available by-product of drinking water facilities using aluminium salts as coagulant was used as the media. Under a hydraulic loading rate of 1.27 m3/m2.d and a range of organic loading rate of 279.4–774.7 g-BOD5/m2.d and 361.1–1028.7 g-COD/m2.d, average removal efficiencies (mean ±SD) of 90.6 ±7.5% for BOD5 and 71.8 ±10.2% for COD were achieved, respectively. P removal was exceptional with average removal efficiency of 97.6 ±1.9% achieved for soluble reactive P at a mean influent concentration of 21.0 ±2.9 mg/l. Overall, the system holds great promise as a novel CWs for simultaneous removal of P and OM, and at the same time, it transforms alum sludge from a waste into a useful material.

The adsorption characteristics of phosphate adsorption on the dewatered alum sludge were identified as a function of pH and ion strengths in solution. In addition, adsorption mechanisms were investigated by conducting batch tests on both the hydrolysis and P-adsorption process of the alum sludge, and making a comparative analysis to gain newer insights into understanding the adsorption process. Results show that the adsorption capacity decreased from 3.5 to 0.7 mg-P/g-sludge when the solution pH was increased from 4.3 to 9.0, indicating that adsorption capacity is largely dependent upon the pH of the system. The results of the competitive adsorption between phosphate and typical anions found in wastewater, such as SO42- and Cl-, onto alum sludge reveal that alum sludge can selectively adsorb phosphate ions. The insignificant effect of SO42- and Cl- on P-adsorption capacity indicates that phosphate adsorption is through a kind of inner-sphere complex reaction. During the adsorption process, the decrease of phosphate concentration in solution accompanied with an increase in pH values and concentrations of SO42-, Cl- and TOC (total organic carbon) suggests that phosphate replaced the functional groups from the surface of alum sludge which infers that ligand exchange is the dominating mechanism for phosphate removal. At the same time, the simultaneous decreases in PO43- and total aluminium concentration in solution indicate that chemical reaction and precipitation are other mechanisms of phosphate removal.

In this study, alum sludge was introduced into co-conditioning and dewatering with an anaerobic digested activated sludge to examine the role of the alum sludge in improving the dewaterbility of the mixed sludge and also in immobilizing phosphorus in the reject water. Experiments have demonstrated that the optimal mix ratio for the two sludges is 2:1 (anaerobic digested sludge: alum sludge; volume basis), and this can bring about 99% phosphorus reduction in the reject water through the adsorption of phosphorus by Al in the sludge. The phosphorus loading in wastewater treatment plants is itself derived from the recycling of reject water during the wastewater treatment process. Consequently, this co-conditioning and dewatering strategy can achieve a significant reduction in phosphorus loading in wastewater treatment plants. In addition, the use of the alum sludge can beneficially enhance the dewaterbility of the resultant mixed sludge by decreasing both the SRF and the CST, due to the alum sludge acting as a skeleton builder. Experiments have also demonstrated that the optimal polymer (Superfloc C2260) dose for the anaerobic digested sludge was 120 mg/l while the optimal dose for the mixed sludge (mix ratio 2:1) was 15 mg/l, highlighting a huge saving in polymer addition. Therefore, from the technical perspective, the co-conditioning and dewatering strategy can be viewed as a “win-win” situation. However, for its full-scale application, integrated cost-effective analysis of process capabilities, sludge transport, increased cake disposal, additional administration, polymer saving etc. should be factored in.

This study investigated an important issue of aluminium (Al) release in a novel reuse of Al-based water treatment sludge (Al-WTS) in constructed wetland system (CWs) as alternative substrate for wastewater treatment. Al-WTS is an inevitable by-product of drinking water treatment plants that use Al-salt as coagulant for raw water purification. It has recently been demonstrated that Al-WTS can be reused as a low-cost phosphorus (P) adsorbent and biofilm carrier in CWs for wastewater treatment. However, to facilitate the large scale application of Al-WTS in CWs as wetland substrate, concerns about Al leaching during its’ reuse in CWs must be addressed as Al is a dominant constituent in Al-WTS. In this study, a desk review of literature on Al release during Al-WTS reuse was conducted. Furthermore, a 42-week Al monitoring was carried out on a pilot field-scale CWs employing Al-WTS as main substrate. Results show that 22 out of the 35 studies reviewed, reported Al release with levels of soluble Al reported ranging from 0.01 to about 20 mg L-1. Monitoring of Al in the pilot field-scale CWs shows that there was Al leaching. However, except for the first three weeks of operation, effluents concentrations of both total- and soluble-Al were all below the general regulatory guideline limit of 0.2 mg L-1. Overall, the study addresses a very vital concern regarding the successful application of Al-WTS in CWs and shows that Al release during such novel reuse is quite low and should not preclude its use.

This paper describes (in a summarised manner) a research attempt to integrate the dewatered alum sludge, a residual by-product of drinking water treatment process, into a constructed wetland (CW) system for the purpose of enhancing the wastewater treatment performance, thus developing a so called alum sludge-based constructed wetland system. A multi-dimensional research project including the batch tests of phosphorus (P) adsorption onto alum sludge followed by the model CWs trials of single and multi-stage CWs, has been conducted since 2004. It has been successfully demonstrated that the alum sludge-based CW is capable of enhanced and simultaneous removal of P and organic matter (in terms of BOD5 and COD), particularly from medium and high strength wastewater. The sludge cakes act as the carrier for developing biofilm for organics removal and also serve as adsorbent to enhance P immobilization. Batch P-adsorption tests revealed that the alum sludge tested possesses excellent P-adsorption ability of 14.3 mg-P/g.sludge (in dry solids) at pH 7.0 with the adsorption favored at lower pH. The results obtained in a 4-stage treatment wetland system suggest that high removal efficiencies of 90.4% for COD, 88.0% for BOD5, 90.6% for SS, 76.5% for TN and 91.9% for PO43--P under hydraulic loading of 0.36m3/m2·d can be achieved. The field demonstration study of this pioneering development is now underway.

Alum sludge refers to the by-product from the processing of drinking water in Water Treatment Works. In this study, groups of batch experiments were designed to identify the characteristics of dewatered alum sludge for phosphorus adsorption. Air-dried alum sludge (moisture content 10.2%), which was collected from a Water Treatment Works in Dublin, was subjected for artificial P-rich wastewater adsorption tests using KH2PO4 as a model P source. Adsorption behaviours were investigated as a function of amount and particle size of alum sludge; pH of solution; and adsorption time. The results have shown that pH plays a major role not only in the adsorption process but also in the adsorption capacity. With regard to adsorption capacity, this study reveals the Langmuir adsorption isotherm being the best fit with experimental data (R2=0.98-0.99). The maximum adsorption capacities range from 0.7 to 3.5mg-P/g when the pH of the synthetic P solution was varied from 9.0 to 4.3, accordingly. The outcome of this study indicated that alum sludge is suitable for use as an adsorbent for removal of phosphate from wastewater.

Excess phosphorus (P) in wastewaters promotes eutrophication in receiving waterways. A cost-effective method such as use of novel low-cost adsorbents for its adsorptive removal would significantly reduce such impacts. Using batch experiments, the intrinsic dynamics of P adsorption by waste alum sludge (an inevitable by-product of drinking water treatment plants) was examined. Different models of adsorption were used to describe equilibrium and kinetic data, calculate rate constants and determine the adsorption capacity. Results indicate that the intraparticle rate constant increased from 0.0075 mg g-1 min-1 at 5 mg L-1 to 0.1795 mg g-1 min-1 at 60 mg L-1 indicating that more phosphate is adsorbed per gram.min at higher P concentration. Further analyses indicate involvement of film and particle diffusion mechanisms as rate controlling steps at lower and higher concentrations respectively. Mass transfer coefficient obtained ranged from 1.7 × 10-6 to 1.8 × 10-8 indicating a rapid transportation of phosphate molecules onto the alum sludge. These results further demonstrates that alum sludge – hitherto thought of as undesirable waste, can be used as novel adsorbent for P removal from wastewater through various applications, thus offsetting a portion of the disposal costs while at the same time improving water quality in sensitive watersheds.

The use of Fenton’s reagent (Fe2+/H2O2) and Fenton-like reagents containing transition metals of Cu(II), Zn(II), Co(II) and Mn(II) for an alum sludge conditioning to improve its dewaterability was investigated in this study. The results obtained were compared with those obtained from conditioning the same alum sludge using cationic and anionic polymers. Experimental results show that Fenton’s reagent was the best among the Fenton and Fenton-like reagents for the alum sludge conditioning. A considerable effectiveness of capillary suction time (CST) reduction efficiency of 47% can be achieved under test conditions of Fe2+/H2O2 = 20/125 mg/gDS (Dry Solids) and pH = 6.0. The observation of floc-like particles after Fenton’s reagent conditioning of alum sludge suggests that the mechanism of Fenton’s reagent conditioning was different with that of polymer conditioning. In spite of the less efficiency in CST reduction of Fenton’s reagent in alum sludge conditioning compared with that of polymer conditioning, is less than that of polymer conditioning. This study provided an example of proactive treatment engineering which is aimed at seeking a safe alternative to the use of polymers in sludge conditioning towards achieving a more sustainable sludge management strategy.

Currently, organic polymers are adopted in alum sludge (aluminium-coagulated drinking water treatment sludge) conditioning. However, there are important concerns regarding the use of these polymers because of the unknown and long-term effects of the potential release of excess polymer to the surrounding environment when the sludge is landfilled. Therefore, as an initiative action, this study aimed at investigating alternative chemical conditioning methods and focused mainly on exploiting Fenton (Fe2+/H2O2) and Fenton-like (Fe3+/H2O2) reagents as the conditioner. Experiments have been conducted to test the effectiveness of Fenton’s reagent (containing the ferrous salts of chloride, sulphate or oxalate), Fenton-like reagent (containing ferric salts of chloride and sulphate) and the coagulation method using FeCl3 for an alum sludge conditioning at a constant hydrogen peroxide and iron salt concentrations of 125 and 20 mg/g DS (dry solids), respectively. The effectiveness on dewaterability of the alum sludge demonstrated that the maximum reduction (%) of SRF (specific resistance to filtration) and CST (capillary suction time) of 74 % and 47 %, respectively, can be obtained when Fenton’s reagent was adopted for sludge conditioning. Such reduction of 64% for SRF and 38% for CST can be achieved when Fenton-like reagents were applied.

This study examined the form, pattern and extractability of phosphorus (P) retained in alum sludge (an aluminium-containing drinking water treatment residual in dewatered cake form), which was used as substrate in laboratory scale constructed wetland systems for P-rich wastewater treatment. Used alum sludge samples from successive depth ranges in the laboratory scale vertical flow constructed wetland systems were examined for KCl extractable P. The samples were also sequentially fractionated into two main categories consisting of readily available P and P forms that are not easily decomposed. The extracted fractions included labile P, microbial P, (Fe + Al) P, humic P, (Ca + Mg) P and residual P. A major proportion of P retained in the used alum sludge was in forms that are not easily decomposed. Of the P forms that are not easily decomposed, the P associated with Ca and Mg accounted for a higher proportion of the inorganic fraction as compared to the P associated with the Fe and Al. The results also show that in most cases, the concentration of the P forms decreased with increasing depth from the topmost surface of the alum sludge in the systems.

In line with the increasing studies on the beneficial reuse of alum sludge from a “waste” into useful raw material, this paper reports an in-depth investigation of the effects of ageing time on the structure and the phosphate adsorption capacity of a dewatered alum sludge obtained from a local drinking water treatment plant in Ireland. During the ageing period from 0 day to up to 18 months, the adsorption capacity of the sludge varied from 21.4 to 23.9 mg-P g-1-sludge at pH 4.3, 14.3 to 14.9 mg-P g-1-sludge at pH 7.0 and 0.9 to 1.1 mg-P g-1-sludge at pH 9.0, respectively, indicating marginal effect of ageing time on such sludge's ability to adsorb phosphate. This result seems conflict with other studies reported in the literature. To reveal such, series of investigations including physicochemical characterization, morphological structure, BET surface area and porous structure of the aged sludge were carried out. All the results conclusively show that ageing time has insignificant effect on the structure and properties of the dewatered alum sludge and thus the phosphate adsorption capacity of the alum sludge remains insignificant change during the ageing.

Waterworks sludge continues to be an inescapable by-product of the potable water treatment process. Accordingly, final disposal of the sludge remains one of the most significant pressing problems for the potable water treatment industry. The possibility of reusing the sludge as a main substrate in a novel constructed wetland system was investigated in this study. Results show that significant phosphorus (P) and other pollutants removal were achieved in the system. With a mean influent BOD5 (5-day biochemical oxygen demand) and COD (chemical oxygen demand) levels of 392.7 mg/l and 579.8 mg/l, respectively, a removal efficiency of 90.6 % and 71.8 %, respectively, was obtained. P removal was however exceptionally high despite the high influent mean P level of 45.3 mg-P/l, which is about 2-3 times the level of P commonly found in sewage. This is attributable to the P adsorption capacity of the alum sludge and this highlights the benefits of its reuse in the system. The paper presents and discusses the findings from a laboratory scale research, which has potential for further large scale implementation.

Phosphorus (P) desorption from P-saturated alum sludge, which was used as main substrate in a novel constructed wetland for wastewater treatment, was studied. Groups of batch experiments were designed and conducted to explore the efficiencies of P extraction using different acids (HCl, HNO3, H2SO4) and bases (NaOH, KOH). The results showed that either acid or base is efficient for P-extraction and the efficiency relied mainly on the concentrations of H+/OH-, not the type of acid or base. Considering the efficiency, price and safety of the acids and bases tested, H2SO4 was chosen as most suitable reagent for P-extraction. A Box-Behnken experimental design based on the response surface methodology (RSM) was applied to evaluate the optimum of H2SO4 extraction. The optimal condition for the mass of sludge, H2SO4 concentration and volume was 0.8 g, 0.063 M and 142 ml, respectively. At such optimal condition, the maximum P-extraction efficiency of 98.2 % was achieved. Additionally, most of the main components of the saturated sludge, such as metals (Al, Ca, Mg, Fe), TOC (total organic carbon) and nitrogen can also be extracted. Overall, the results supported that H2SO4 seems to be an efficient and cost-effective reagent among all the reagents tested for P-saturated alum sludge treatment.

This research assessed the use of gypsum (CaSO4.2H2O) as a skeleton builder for sludge dewatering since polymer conditioning of sludge affected only the rate of water release, not the extent of dewatering. The use of gypsum as a physical conditioner, in association with a polymer, could improve sludge filterability. More significantly, gypsum serves as a skeleton builder, forming a permeable and rigid lattice structure that can remain porous under high positive pressure during the compression step after the cake growth of the filtration, thereby maintaining the size of the micro-passages through which water is expressed. Experiments using a high pressure cell apparatus showed that a further decrease of two to seven percent of the equilibrium moisture content of the sludge cake was achieved, for sludge thicknesses for dewatering of 1 to 10 cm, by the addition of gypsum with 60% of the original sludge solids when compared to the single polymer conditioning. The importance of the addition of gypsum in alum sludge dewatering is not only the improvement in the extent of dewatering, but also the potential application of transforming dewatered alum sludge from ‘waste’ for landfill to useful ‘fertilizer’ or to be used as filter medium/adsorbent for wastewater treatment engineering.

This paper is a research brief highlighting the development of a novel alum sludge based constructed wetland system, aimed at reducing the pollution effects of agricultural run-offs. Alum sludge is a by-product of drinking water treatment plants where aluminium sulphate is used as coagulant. The alum sludge cakes were used as the main support matrix in the constructed wetland system, having been shown to have a greater capacity to retain contaminants, especially phosphorus, through sorption phenomena, ionic exchange and other physico-chemical processes. The dewatered alum sludge cakes proved beneficial in enhancing and sustaining phosphorus removal in the system, while the removal of organics is thought to be enhanced by the tidal flow mechanism employed. Overall removal of 82.3 + 3.5% (BOD5) and 85.5 + 2.1 % (COD) were achieved in the system

The objective of this study was to assess the suitability of statistical and the k-C* models to projecting treatment performance of constructed wetlands by applying the models to predict the final effluent concentrations of a pilot field-scale constructed wetlands system (CWs) treating animal farm wastewater. The CWs achieved removal rates (in g/m2.d) ranging from 7.1-149.8 for BOD5, 49.8-253.8 for COD and 7.1-47.0 for NH4-N. Generally, it was found that the statistical models developed from multiple regression analyses (MRA) were stronger in predicting final effluent concentrations than the k-C* model. However, both models were inadequate in predicting the final effluent concentrations of NO3-N. The first-order area-based removal rate constants (k, m/yr) determined from the experimental data were 200.5 for BOD5, 80.1 for TP and 173.8 for NH4-N and these indicate a high rate of pollutant removal within the CWs.

Organic polymers have long been used as sludge conditioners to improve its dewaterability in sludge management practice. Although polymers can bring about a great dewatering performance of the sludge, their potential health related risk remains unknown regarding their residual in dewatered sludge cakes in the environment when the sludge is finally disposed as landfill especially in long term point of view. For this regard, as an initiative action, Fenton (Fe2+/H2O2) and its related reagents were tested in this study as potential alternative an alum sludge conditioners for the purpose of eliminating the perceived long term risk associated with polymer residual in the environment.

Problems concerning the management and utilization of sludge derived from water treatment processes are still not fully solved. A common approach is direct discharge to a landfill site. This study provides experimental data to demonstrate the effectiveness of a combination of gypsum (CaSO4x2H2O) and an organic polymer in alum sludge conditioning and dewatering. Experimental results demonstrated that the filterability of dually conditioned alum sludge was significantly improved by the addition of gypsum at a 1:1 ratio (WT/WT) to original sludge solids. Dewatering tests showed that a further decrease of almost seven percentage of sludge cake equilibrium moisture content was achieved by the involvement of gypsum compared to the situation of single polymer conditioning. The importance of this study lies in the possible application of dewatered alum sludge to land use or as a filter medium in constructed wetland for wastewater treatment, providing a positive solution to the problem of alum sludge disposal.

Dewatered alum sludge, a widely generated by-product of drinking water treatment plants using aluminium salts as coagulants was used as main substrate in a pilot on-site constructed wetland system treating agricultural wastewater for 11 months. Treatment performance was evaluated and spreadsheet analysis was used to establish correlations between water quality variables. Results showed that removal rates (in g/m2.d) of 4.6-249.2 for 5 day biochemical oxygen demand (BOD5), 35.6-502.0 for chemical oxygen demand (COD), 2.5-14.3 for total phosphorus (TP) and 2.7-14.6 for phosphate (PO4-P) were achieved. Multiple regression analysis showed that effluent BOD5 and COD can be predicted to a reasonable accuracy (R2=0.665 and 0.588, respectively) by using input variables which can be easily monitored in real time as sole predictor variables. This could provide a rapid and cheap alternative to such laborious and time consuming analyses and also serve as management tools for day-to-day process control.

This research has two eventual goals: (1) To optimize performance of subsurface constructed wetlands for removal of phosphorus (P) (2) To demonstrate that dewatered alum sludge (a by-product), can be reused as a constructed wetland substrate. To achieve these, alum sludge from a water treatment plant was characterized and used as main substrate in four experimental vertical sub-surface flow constructed wetland systems treating dairy farm wastewater. Results show that the alum sludge has suitable hydraulic characteristics (uniformity coefficient = 3.6) for use as a substrate, and in the batch studies, up to 48.6mg-P was removed by 1g of the alum sludge at a P concentration of 360mg-P/l and a dosage of 5g/l. Results from the experimental systems highlight the significant P removal ability of the alum sludge. However, the inclusion of pea gravel at the infiltrative surface of some of the systems had a negative effect on the P removal performance. Sequential P-fractionation results show that there was no significant increase in the easily extractable P, but for total P, there was significant increase, although this was found to decrease with depth. This study shows that the novel use of dewatered alum sludge can bring about high P removal in vertical subsurface flow constructed wetland systems.