Patent Publication Number: US-10315943-B2

Title: Methods and systems for pH treatment and extraction of leachable resources and pollutants from sludge

Description:
RELATED APPLICATION 
     This application claims priority from U.S. provisional application No. 62/150,095 of Venkatesan and Halden, filed Apr. 20, 2015, entitled “Methods and Systems for pH Treatment and Extraction of Leachable Resources and Pollutants from Sludge,” which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to methods and systems for pH treatment and extraction of leachable resources and pollutants from sludge. 
     BACKGROUND 
     Continuing growth of the U.S. population is creating an increased demand for wastewater treatment capacity. Consequently, the amount of municipal sewage sludge (MSS) produced will increase in the foreseeable future. In 2004, the U.S. Environmental Protection Agency (U.S. EPA) estimated that approximately 6.5 million tonnes of MSS are produced each year in the U.S. (NEBRA 2007). 
       FIG. 1  shows the different categories and magnitudes of U.S. MSS uses. A first area  10  (farmlands), a second area  12  (horticulture and landscaping) and a third area  13  (land restoration silviculture) represent beneficial land applications. A fourth area  14  (landfill), a fifth area  16  (incineration) and a sixth area  17  (surface disposal only) represent disposal options. As shown, 55% of MSS is estimated to be applied on land for beneficial purposes, totaling approximately 3.6 million tonnes of MSS each year. Treated MSS fit for land application (also termed biosolids) is rich in organic carbon content and nutrients that can improve soil properties, crop productivity and fertility (Smith 1995, Wang et al. 2008). A Life Cycle Assessment (LCA) performed on different treatment and disposal scenario of MSS showed that the combination of anaerobic digestion and agricultural land application was the most viable option creating lower costs and consuming less energy compared to other treatment and disposal options (Suh and Rousseaux 2002). Landfilling of MSS is becoming difficult due to limited land availability, increased compliance costs, and concerns regarding leachate and greenhouse gas emissions from methane production (Wang et al. 2008). Incineration of MSS, on the other hand, involves high operational costs and concerns over toxic flue gas emissions (Lundin et al. 2004). Recent efforts in using MSS as a resource for energy production and nutrient recovery have increased the beneficial value of MSS (Rulkens 2007, Wang et al. 2008). Beneficial reuse of MSS already is applied to over half of the total U.S. MSS mass and this trend is expected to increase further in the foreseeable future. 
     However, a notable downside to the beneficial reuse of MSS is the presence of persistent (P), bioaccumulative (B), and toxic (T) chemicals, toxic metals and pathogens in MSS, all known to pose significant environmental and human health concerns (Chaney et al. 1996, Dowd et al. 2000, Gerba et al. 2002, National Research Council 2002). 
     The number of hazardous chemicals detected in MSS is considerable and constantly increasing (U.S. EPA 2009, Venkatesan et al. 2015, Venkatesan and Halden 2014a,b,c). Currently, the U.S. EPA has established standards for nine toxic compounds in land-applied MSS, all belonging to the inorganic group of metals, including As, Cd, Cu, Hg, Mo, Ni, Pb, Se, and Zn (40 CFR Part 503 of US Federal Regulations). The Part 503 regulation focuses on the presence of (a) nine metals, (b) microbial pathogens, and (c) MSS unwanted attraction to disease vectors as a basis for determining MSS quality. 
     Furthermore, in addition to the aforementioned concerns, the presence of significant quantities of organic contaminants of emerging concern (CECs) poses a newly recognized threat to the practice of land application of MSS (Chaney et al. 1996, Clarke and Smith 2011, Topp et al. 2008, Venkatesan et al. 2015). Many of these contaminants are known endocrine disruptors, carcinogens and potent toxicants to aquatic organisms. In addition, an emerging concern is the presence of significant quantities of important antibiotics in MSS (e.g. ciprofloxacin) that is known to promote antibiotic resistance in human pathogens even when present at nontherapeutic concentrations (Martins da Costa et al. 2006, Pruden 2013, and Reinthaler et al. 2003). 
     Prior screening by Arizona State University (ASU) for 239 contaminants in nationally representative samples of MSS collected from more than 160 U.S. wastewater treatment plants (WWTPs), showed the presence of 130 CECs in MSS (Venkatesan et al. 2015, Venkatesan et al. 2014a, 2014b). These included pharmaceuticals and personal care products (PPCPs), brominated flame retardants (BFRs), perfluorinated chemicals (PFCs), 4-nonylphenol and its ethoxylates (NP and NPEOs), as well as hormones, polybrominated dioxins and furans (PBDD/Fs), and nitrosamines. Chemicals detected were calculated to contribute about 0.04-0.15% of the total dry mass of MSS produced in the U.S. annually, a mass equivalent to 0.4-1.5 g/kg of dry sludge or about 4,700 tonnes (range: 2,600-7,900 tonnes) of chemicals annually. However, this estimate is conservative with respect to mass (i.e., lower than the true value), since other industrial organic chemicals known to occur in MSS (e.g., linear alkylbenzene sulfonates, PCBs, etc.) were not included in the screening. The lack of transformation of these contaminants during optimized biological wastewater treatment and sludge treatment processes (e.g., activated sludge treatment followed by anaerobic digestion), indicate a strong persistence and resistance to biotransformation of these compounds upon release into the environment (Venkatesan and Halden 2014a). 
     Another important concern is leaching of contaminants from land applied MSS. Recent studies have shown contamination of groundwater, surface water, and uptake of CECs by agricultural crops from soils amended with MSS (Clarke and Smith 2011, Lapworth et al. 2012, Sepulvado et al. 2011, Wu et al. 2010, Xia et al. 2010). NP and NPEOs, PFCs and PPCPs have been shown to leach from MSS at significant concentrations (Edwards et al. 2009, La Guardia et al. 2001, Lindstrom et al. 2011, and Sepulvado et al. 2011). PFC concentrations in well water and surface water resulting from contaminant leaching from nearby fields amended with MSS were shown to be in excess of U.S. EPA&#39;s health advisory level for drinking water (Lindstrom et al. 2011). This brings up two important questions addressed by the inventors in the solution presented herein: First, what fraction of the estimated contaminant load of 4,700 tonnes can readily leach from land-applied MSS to contaminate our water resources? And, secondly, is there a straightforward technology that can remove this leachable fraction of the contaminant mass from MSS prior to disposal on land? Current safety assessments of land-applied MSS are incomplete, as this information is not available. The methods and systems presented herein address this knowledge gap and introduce a straightforward process for removing the leachable fraction of contaminants from MSS to prevent adverse environmental and human health effects upon MSS recycling on land. 
     The present invention provides novel solutions for the deficiencies inherent in methods like those described above. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     This summary is provided to introduce, in a simplified form, a selection of concepts that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     A pH treatment and extraction of leachable resources and pollutants from sludge is provided. A volume of wastewater sludge is initially dewatered followed by adding pH adjusting agents to the dewatered sludge to remove acid/base-soluble contaminants. The acid/base-soluble contaminants are flushed out or dewatered from the treated product. Treated sludge can then be sent to storage and held for beneficial uses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the novel features of the invention are set forth with particularity in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings, in which: 
         FIG. 1  graphically illustrates municipal sewage sludge uses and disposal modes in the United States for the year 2004 by category and magnitude shown in percent (Data extracted from NEBRA, 2007). 
         FIG. 2A - FIG. 2B  graphically illustrate comparisons of ionic and neutral contaminants detected in nationally representative U.S. municipal sewage sludge samples by number ( FIG. 2A ) and by mass ( FIG. 2B ). Total value (4700 tonnes/year) represents the estimated amount of CEC mass accumulating in MSS each year. 
         FIG. 2C  graphically represents the breakdown of the number of ionic contaminants detected in MSS by type. 
         FIG. 3A - FIG. 3D  show modeling of relative fraction of IOCs associated with the solids content of MSS as a function of pH. 
         FIG. 4  graphically shows estimated percent desorption of IOCs (n=52) with a combination of acidic (&lt;3) and basic (&gt;12) pH treatment as a function of initial (ambient) sludge pH. E.g., sludge with initial pH between 6 and 8 is ideal to treat and remove &gt;40% of IOCs prior to land-application. 
         FIG. 5A  graphically shows estimated nationwide removal of ionizable organic contaminants from sewage sludge (initial pH of 7.5) by pH treatment. Error bars represent maxima and minima. 
         FIG. 5B  shows projection of IOCs removed nationwide from land-applied MSS via proposed pH treatment. ‘n’ represents the number of chemicals modeled. 
         FIG. 6  illustrates a schematic of a pH treatment implemented in a full-scale wastewater sludge treatment system. 
     
    
    
     In the drawings, identical reference numbers identify similar elements or components. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following disclosure describes systems and methods for treatment of a volume of wastewater sludge. Several features of methods and systems in accordance with example embodiments are set forth and described in the Figures. It will be appreciated that methods and systems in accordance with other example embodiments can include additional procedures or features different than those shown in the Figures. Example embodiments are described herein with respect to a method for pH treatment and extraction of leachable resources and pollutants from sludge. However, it will be understood that these examples are for the purpose of illustrating the principles, and that the invention is not so limited. Additionally, methods and systems in accordance with several example embodiments may not include all of the features shown in the Figures. 
     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.” 
     Reference throughout this specification to “one example” or “an example embodiment,” “one embodiment,” “an embodiment” or combinations and/or variations of these terms means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     Definitions 
     Generally, as used herein, the following terms have the following meanings when used within the context of wastewater sludge treatment: 
     A “sample” as used herein refers to material, such as environmental material obtained from a remediation site that is suspected of containing, or known to contain, contaminants of concern. 
     “Contaminants” include chemicals and other compounds hazardous to human health, such as pharmaceuticals and other chemicals of concern. 
     Contaminants of Concern in Sewage Sludge 
     Among 429 organic chemicals screened for in U.S. MSS samples, 312 or 73% of the chemicals were detected. The sum of average concentration of 312 chemicals was 727 (sum of minimum: 412; sum of maximum: 1235) mg/kg dry weight (dw) of sludge (Venkatesan and Halden 2016). 
     Referring now specifically to  FIG. 2A - FIG. 2C , comparisons of ionic and neutral contaminants detected in nationally representative U.S. municipal sewage sludge samples by number ( FIG. 2A ) and by mass ( FIG. 2B ) are graphically shown where n represents the number of chemical compounds.  FIG. 2C  represents the breakdown of the number of ionic contaminants detected in MSS by type (Venkatesan and Halden, 2016). 
     Out of the 312 contaminants, only 71 (23%) chemicals were identified as ionizable ( FIG. 2A ). Interestingly, these chemicals made up 82% of the total mass of chemicals sequestered in U.S. MSS [594 (range: 440 to 945) mg/kg dw] ( FIG. 2B ). Detected ionizable compounds included 57 PPCPs, 12 PFCs, one surfactant and one pesticide, and the annual load of IOCs in U.S. MSS are estimated to exceed 3800 t annually. 
     Referring jointly to  FIG. 2A  and  FIG. 2B , seventy-one compounds  30  (23%) were ionizable compounds, contributing 82% (3850 tonnes) to the total contaminant mass in MSS. The remaining 77% were neutral compounds  32 . As specifically shown in  FIG. 2B , the neutral compounds contributed 18% (850 tonnes) to the total contaminant mass in MSS, annually. 
     Of this quantity, 55% is land-applied which means more than 2000 t of ionizable compounds per year is applied on U.S. soils. Considering a typical average biosolids land application rate of 16.8 t/ha (7.5+2.5 dry U.S. tons per acre), this value translates to about 10 (range: 7.4 to 16) kg of IOCs applied per hectare of agricultural land. A breakdown of important class of chemicals falling into this category shows that antibiotics alone represent 20% of total IOCs by number ( FIG. 2C ). 
     Referring now specifically to  FIG. 2C , identified ionizable compounds included 57 PPCPs, 12 PFCs and the surfactant nonylphenol. More specifically identified are antibiotics  38  representing 20%, PFAS  40  representing 17%, analgesics  42  representing 9%, antidepressants  44  representing 7%, antianginals  46  representing 4%, antibacterials  48  representing 4% and other compounds  50  representing 39% of the total identified ionizable compounds. 
     One of the factors that influence water solubility and mobility (leachability) of organic contaminants from MSS is the pH of the environment. This is especially the case for ionizable contaminants that are prone to leach out of MSS under acidic or basic pH conditions, depending on the compounds&#39; physical-chemical properties (e.g., ionization constants). The sorption behavior of neutral compounds on the other hand is constant, irrespective of pH. Hence, adjusting the pH of MSS has the potential to mobilize these ionizable organic contaminants (IOCs) that can then be extracted by flushing and removed prior to land application. 
     Adjustment of sludge pH to a value of greater than 12 via lime addition is a common process for sludge stabilization and has been employed in WWTPs for more than four decades (U. S. EPA 1979). However, this treatment is currently focused only on a reduction of the pathogen content of MSS. The secondary, major benefit of mobilizing IOCs during lime stabilization has not yet been evaluated and utilized to date and, as practiced today, the lack of sludge flushing after lime treatment prevents successful removal of pollutants prior to MSS release to the environment. Moreover, certain IOCs require an acidic pH to get mobilized; a process that is not commonly used for sludge stabilization. In contrast to known methods, the method described herein below will reduce the leachable fraction of IOCs in MSS by pH adjustment and extraction prior to land application of MSS. 
     Important additional benefits of pH adjustment are (i) increased mobility of toxic metals at low pH that could be extracted along with certain IOCs, and (ii) decreased pathogen content of MSS due to acidic or basic stabilization. Also, a combination of acidic followed by basic treatment will produce high amount of heat from exothermic reaction or heat of hydration, that can cause thermal inactivation of pathogens (See U.S. Pat. No. 5,385,673 which is incorporated by reference). Both metals and pathogens are currently regulated in MSS by the U.S. EPA and thus the proposed project has the potential to further reduce the burden of toxic metals and pathogens in sludge to produce MSS of exceptional quality, as defined in 40 CFR Part 503. The beneficial value of MSS currently is diminished by the presence of significant amounts of contaminants. Yet, in sustainability terms, land application of MSS is both cost-effective and prudent to return scarce nutrients like phosphorus back to agricultural soils. Hence, the disclosed method will make land application a more sustainable and safer practice, and provide a cost-effective and straightforward treatment process that can be implemented easily in existing sludge treatment systems in U.S. WWTPs and worldwide, to significantly improve the quality of MSS destined for land application. Thereby, precious soil and water resources are protected from unnecessary contamination. 
     Hypothetical Examples 
     A modified mathematical model using the chemical sorption equation was used to estimate the fraction of IOCs that can be potentially desorbed from MSS by the proposed pH treatment system (Venkatesan and Halden 2016). In order to account for feasibility of the process, only IOCs featuring a pK a  value within 3 and 10 were considered for this model. The fraction of IOCs sorbed onto MSS (f s ) was modeled for any given value of pH-adjusted distribution coefficient (D) using the following modified sorption equation from Fjeld et al. 2007: 
                     f   s     =       D   ⁢           ⁢   ρ   ⁢           ⁢     s   f         1   +     D   ⁢           ⁢   ρ   ⁢           ⁢     s   f                   Eq   .           ⁢   1               
Where ρ is the density of MSS, s f  is the solids fraction of sludge (the product of ρ and s f  being the MSS solids concentration) and assuming that the contaminants partition onto the total (organic and inorganic) solids fraction. A typical value of ρ=1.55 g/cm3 was used for digested MSS (O&#39;Kelly 2005). A typical value of 10% of solids (s f =0.1) in treated MSS for disposal was used for the model. In the case of chemicals with multiple pK a  values, only values within 3 and 10 were considered for modeling purposes. Average pH of MSS can vary between 6 and 8 depending on the type of sludge processing technique (e.g. lime treated sludge can have pH &gt;9). A typical pH of 7.5 was used as the reference pH of MSS to estimate the relative amount of sorption/desorption of contaminants as a function of sludge pH change.
 
     Out of the 71 detected IOCs, 52 chemicals had at least one of their pK a  value within the range of 3 to 10 and their sorption behavior was modeled using equation 1. The relative fraction sorbed is defined here as the amount of chemical sorbed on to the sludge at a given pH relative to the initial amount sorbed to the sludge at ambient pH value (in this case pH of 7.5). As shown in  FIG. 3B  and  FIG. 3C , the sorption of acids decreases and of bases increases with increasing pH. The sorption behavior is complex for chemicals with multiple pK a  values, and in general peaks in the neutral pH range (7), and drops with increasing acidic or basic pH conditions. 
     This model was applied to find that between 36 and 85% of the mass of individual classes of IOCs potentially can be desorbed due to ionization forced by adjustment of the typical pH value of 7.5 in MSS. However, this model accounts only for hydrophobic sorption of contaminants and does not include other mechanisms like electrostatic attraction that explains sorption behavior of many ionized compounds (e.g., PFCs and antibiotics). An overall desorption of 30% of IOCs by mass at acidic pH (&lt;3) and 31% of IOCs at alkaline pH (&gt;12) could be achieved by pH adjustment. A combination of acidic and basic pH treatment could result in desorption of &gt;40% of IOC mass detected in MSS. This estimate is conservative, as it considers only IOCs with pK a  values between 3 and 10; whereas IOCs featuring a very low pK a  value and those that electrostatically sorb onto solids also potentially can desorb following pH adjustment, as demonstrated for pharmaceuticals and PFCs (Chen et al. 2011, Wang and Shih 2011). 
     Referring now to  FIG. 3A - FIG. 3D  where modeling of relative fraction of IOCs associated with the solids content of MSS as a function of pH is graphically illustrated.  FIG. 3A  shows sorption to solids in MSS of IOCs (featuring pK a  values between 3 and 10) detected previously by the ASU team in U.S. MSS. Additional panels show sorption characteristics of individual classes of MSS constituents: basic ( FIG. 3B ), acidic ( FIG. 3C ) and zwitterionic ( FIG. 3D ) compounds detected in MSS, where ‘n’ represents the number of contaminants in each class. Dotted vertical line 401 indicates the typical MSS pH value of 7.5. Percentage values shown are the estimated maximum desorption of IOCs from MSS due to ionization as a function of either increasing or decreasing the pH away from the default of 7.5. (relative to the initial amount sorbed on to the sludge at pH 7.5). 
     Referring now to  FIG. 4 , estimated percent desorption of ionizable organic compounds (IOCs) (n=52) with a combination of acidic (&lt;3) and basic (&gt;12) pH treatment as a function of initial (ambient) sludge pH. E.g., sludge with initial pH between 6 and 8 is ideal to treat and remove &gt;40% of IOCs prior to land-application is graphically shown. 
     The model results show that 12 IOCs feature desorption of &gt;50% with the proposed pH adjustment of MSS: three antibiotics (clarithromycin, enrofloxacin, norfluoxetine), three antihistamines (atorvastatin, amlodipine, diphenhydramine), two analgesics (hydrocodone, oxycodone), one anti-inflammatory (ibuprofen), one antidiabetic (gemfibrozil), one antifungal (thiabendazole) and one diuretic (triamterene). Antihistamines (n=3) were shown to feature maximum desorption of 64% from MSS with the proposed pH adjustment, followed by anti-inflammatory drugs (n=2) and analgesics (n=6) at 51% and 33%, respectively. Many of these contaminants have been shown to occur in runoffs, tile water and groundwater near MSS-applied agricultural lands (Edwards et al., 2009; Gottschall et al., 2012). 
     Referring now to  FIG. 5A , estimated nationwide removal of ionizable organic contaminants from sewage sludge (initial pH of 7.5) by pH treatment. Error bars represent maxima and minima is graphically shown. Bar  501  plots removal with acid treatment having &lt;pH 3, where n=42. Bar  502  plots removal with base treatment having &gt;pH 12 where n=15. Bar  503  plots removal with combined treatments where n=52. The letter n represents the number of ionizable organic compounds (IOCs). 
     One important limitation of the present model is that it does not account for electrostatic sorption of chemicals. Antibiotics, an important class of contaminants falling into this category, have been detected widely in the environment including in MSS (n=14), even when their sorption coefficients are low (Batt et al., 2006; Kummerer, 2003; McClellan and Halden, 2010). Leaching of antibiotics from soils has been identified as one of the most important pathways for contamination of water bodies (Overcash et al., 2005; ter Laak and Gebbink, 2006; Thiele-Bruhn, 2003). Batch sorption experiments conducted in the laboratory showed that the interaction between antibiotics and soil particles were governed predominantly by solution pH (Hari et al., 2005; ter Laak and Gebbink, 2006). Transport of the antibiotic ciprofloxacin (pK a,1 =6.2; pK a,2 =8.8) through a column of porous media was studied at two different solution pH of 5.6 and 9.5 (Chen et al., 2011). The column showed no breakthrough of ciprofloxacin when flushed with a solution of pH 5.6 (lower than its pK a,1  value of 6.2). However, at a pH of 9.5 (greater than its pK a,2  value of 8.8) ciprofloxacin showed high mobility and about 93% of the compound was observed to desorb from the porous media and left the column after flushing with four pore volumes of deionized water (Chen et al., 2011). Interestingly the present sorption model estimated only 7% desorption rate for ciprofloxacin from MSS after acidic (&lt;3) and basic (&gt;12) pH treatment. This huge difference (92%) between the model and experimental data is due to the exclusion of electrostatic attraction of ciprofloxacin in the present model and hence underestimating the sorption capability of the compound. One of the major concerns with regards to environmental presence of antibiotics is the promotion of antibiotic resistance genes in microorganisms (Pruden, 2013; World Health Organization, 2010). Electrostatic attraction has also been observed for PFCs. It has been shown that the sorption of ionized PFCs is due to electrostatic attraction with the sorbent (Johnson et al., 2007; Wang and Shih, 2011). The pH dependence of the sorption characteristics of perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA), have been studied in the past. These PFCs having very low pK a  values (&lt;0.52) and high solubility in water have been detected in MSS at parts-per-billion levels. Sorption of PFOS to five different materials (goethite, kaolinite, Ottawa sand, iron oxide-coated sands, and sediment from Lake Michigan) showed notable decreases at increased pH, suggesting electrostatic interactions as a prominent parameter governing PFOS partitioning and mobility (Johnson et al., 2007). A similar observation of reduced sorption of PFOA and PFOS to alumina was made as a function of increased pH and ionic strength (Wang and Shih, 2011). There are several other compounds in sludge that may behave similarly to antibiotics and PFCs, and hence are not captured by the model. This suggests that the desorption estimates from the model are very likely to be less than the true value, that may result in an overall higher removal of contaminants from MSS via pH treatment. 
     An important additional benefit of pH adjustment is a decrease in the pathogen content of MSS due to acidic or basic stabilization. A combination of acidic followed by basic treatment will produce a high amount of heat from the exothermic reaction or heat of hydration, thus facilitating thermal inactivation of pathogens (Fergen, 1995). In addition to organic compounds, it has been shown that metal mobility also depends on several factors including pH, organic matter content, type of sludge and soil, cation-exchange capacity, etc. Heavy metal mobility in sludge-amended soils at different pH levels has been studied in the past (Nikolaidis and Chheda, 2001; Tackett et al., 1986). It was reported that heavy metals, including Cr, Cu, Pb, Ni, Zn and Cd, featured increased mobility and leaching from MSS as a function of decreased pH. Additionally, batch experiments showed removal of heavy metals when contaminated MSS was treated with 20% (v/v) of sulfuric acid, with removal rates of 74% for Ni, 99% for total Cr, 86% for Cu, 11% for Pb and 72% for Zn (Stylianou et al., 2007). Both metals and pathogens are currently regulated in MSS by the U.S. EPA and thus the proposed pH treatment has the potential to further reduce the burden of toxic metals and pathogens in sludge to produce MSS of enhanced quality. 
     The proposed pH treatment system could be implemented in WWTPs after the initial dewatering step of sludge processing (e.g. anaerobic or aerobic digestion process) ( FIG. 6 ). The total concentration of the 52 modeled IOCs in nationally representative MSS samples was 30 mg/kg dw. This concentration translates to 195 t of IOCs in U.S. MSS annually. The model estimates a removal of 40±16 t annually from U.S. MSS; i.e., between 12 and 29% of the selected IOCs could be potentially desorbed and extracted via pH treatment. However, this range is conservative (i.e., lower than the true value) as indicated above due to the exclusion in the model of electrostatic sorption of chemicals and other contaminants with pK a  outside the range of 3 to 10. A prior study that performed Toxicity Leaching Characteristic Procedure (TCLP) showed that an average 2% of NP and NPEOs leached from sewage sludge (La Guardia et al., 2001). Using this value on the national baseline level of NP in U.S. MSS suggests about 69 t/y of NP and NPEOs could leach from land-applied MSS; a fraction of which could be prevented via the proposed pH treatment. In addition, pH treatment could aid in the removal of toxic and regulated metals (Babel and del Mundo Dacera, 2006; Yoshizaki and Tomida, 2000). Using the percentage removal of seven regulated metals (As, Cd, Cu, Pb, Hg, Ni, Zn) reported in the literature, it was estimated that about 1 g per kg of dry sludge or 7000±2300 t annually could be mobilized and removed from U.S. MSS via pH treatment. 
     A comparison of PPCPs data between the 2001 and 2007 national sewage sludge survey revealed that the concentrations of contaminants remained fairly constant over five years (McClellan and Halden, 2010). Hence, assuming similar concentration trends of the selected IOCs in MSS in the future and considering a per capita MSS production of 22.5 kg/per person/year estimated for the year 2007, the removal of contaminants via pH treatment was extrapolated to the year 2060 based on population projections from the U.S. Census Bureau (as shown in  FIG. 5B ). Projections shows up to 30 t of leachable IOCs can potentially be avoided from land-applied MSS via the proposed pH treatment. The estimated removal of IOCs provided above assumes a single extraction or flushing event of pH-treated MSS to remove contaminants. The removal can be further enhanced through sequential extraction of MSS. 
     Referring now to  FIG. 5B , projection of IOCs removed nationwide from land-applied MSS via proposed pH treatment. ‘n’ represents the number of chemicals modeled. Curve  510  indicates a projection in IOC&#39;s in biosolids if pH treatment is applied. Curve  512  indicates a projection in IOC&#39;s in biosolids if pH treatment is not applied. The double-headed arrow  520  indicates the difference as the amount of leachable fraction removed. 
     Wastewater Sludge Treatment System 
     Referring now to  FIG. 6 , a flow schematic of a pH treatment implemented in a full-scale wastewater sludge treatment system is illustrated. A wastewater sludge treatment system  400  includes a sludge treatment system  402  fluidly connected to a storage tank for holding enhanced biosolids  406  and a sidestream flow  408 . The sludge treatment system  402  may comprise a digestion tank holding wastewater sludge in suspension, for example. The reactor  404  further comprises an acidification stage  420  and a lime treatment stage  422  both fluidly coupled to the sidestream flow  408 . 
     In operation a volume of treated wastewater sludge from the sludge treatment system  402  undergoes an initial dewatering in stage  440  before being delivered to the reactor  404 . After the initial dewatering the dewatered product may be transported to the acidification stage  420  followed by the lime treatment stage  422 . In the acid treatment stage  420 , extraction agent is added to the product to remove acid-soluble contaminants. The acid-soluble contaminants are removed either by flushing out the contaminants from the product using a flushing agent (e.g., treated wastewater) at stage  426  or by dewatering of the sludge to remove acid soluble concentrated contaminants and resources  426 . The concentrated process streams  426  and  424  can be reacted for heat generation derived from the thermal energy of neutralization. The combined process flows  426  and  424  can be processed and used for chemical resource recovery prior to disposal of the liquid stream  408  or its return to the headworks of the treatment operation. Lime treatment is applied at stage  422  to remove base-soluble contaminants which are also either flushed out of the product at stage  424  into the combined flow  408 , or reacted for energy and value recovery, such as neutralization as described above. Metals and other resources contained in sludge are known to be of significant monetary and resource value (Westerhoff et al., 2015). Beyond the removal of undesirable contaminants of sludge prior to utilization, there is an opportunity to recover latent heat and elemental, molecular, and biological resources. The reactor process described above may be repeated through multiple cycles to increase the efficiency of pollutant and resource depletion of sludge. After flushing/dewatering, the product is then transported to a tank for storage or directly enters utilization  406 . 
     Examples of flushing agents include treated wastewater, treated effluent, drinking water, tap water, solvents, combinations thereof and the like. Dewatering may be accomplished using a belt press, centrifuge or equivalents. Acidification materials are taken from the group of organic acids, including citric acid and oxalic acid, and from the group of inorganic acids, including sulfuric, nitric and hydrochloric acid, as well as combinations of these. 
     The invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles of the present invention, and to construct and use such exemplary and specialized components as are required. However, it is to be understood that the invention may be carried out by different equipment, and devices, and that various modifications, both as to the equipment details and operating procedures, may be accomplished without departing from the true spirit and scope of the present invention. 
     REFERENCES 
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