Revisiting Chemically Enhanced Primary Treatment of Wastewater: A Review

: Chemically enhanced primary treatment (CEPT) is a process that uses coagulant and / or ﬂocculant chemicals to remove suspended solids, organic carbon, and nutrients from wastewater. Although it is not a new technology, it has received much attention in recent years due to its increased treatment capacity and related beneﬁts compared to the conventional primary treatment process. CEPT involves both physical and chemical processes. Alum and iron salts are the commonly used coagulants in CEPT. Several types of anionic, cationic, and uncharged polymers are used as ﬂocculants, where poly aluminum chloride (PACL) and polyacrylamide (PAM) are the widely used ones. Some of the coagulants and ﬂocculants used may have inhibitory and / or toxicity e ﬀ ects on downstream treatment and recovery processes. There has been an increasing amount of work on the treatment of wastewaters from various sources using CEPT. These wastewaters can range from municipal / domestic wastewater, combined sewer overﬂow, landﬁll leachate, cattle manure digestate to wastewaters from textile industry, pulp and paper mill, slaughterhouse, milk processing plant, tannery and others. In recent cases, CEPT is employed to enhance carbon redirection for recovery and substantially reduce the organic load to secondary treatment processes. CEPTs can remove between 43.1–95.6% of COD, 70.0–99.5% suspended solids, and 40.0–99.3% of phosphate depending on the characteristics of wastewater treated and type of coagulants and / or ﬂocculants used. This article reviews the application, chemicals used so far, removal e ﬃ ciencies, challenges, and environmental impacts of CEPT.


Introduction
Primary wastewater treatment has been identified as a key area of research in improving the sustainability of wastewater treatment processes [1]. Conventional primary treatment processes' major focus has been suspended solids and phosphorus removal. In chemically enhanced primary treatment (CEPT), the major objective is to remove pollutants such as organic carbon (carbon redirection) and nutrients and concentrate it in the sludge produced, which can be regarded as valuable resources to increase energy recovery. There has been an increasing amount of work on the treatment of wastewaters from various sources using CEPT. These wastewaters can range from municipal/domestic wastewater, combined sewer overflow, landfill leachate, cattle manure digestate to wastewaters from textile industry, pulp and paper mill, slaughterhouse, milk processing plant, tannery and others. There is some evidence suggesting the use of CEPT to treat industrial wastewaters that are difficult to treat. For example, among different physical, chemical, biological, and advanced chemical oxidation technologies, chemical coagulation and flocculation is still a cost-comparative alternative for the treatment of industrial textile wastewaters and is widely practiced by small to large scale industries [2].

Application and Efficiencies of CEPT Processes
Harleman and Murcott [9] provided CEPT application examples from Mexico City, Southern California, Hong Kong, Sao Paulo, and Rio de Janeiro. For instance, in Hong Kong, over 75% of sewage (~2.2 million m 3 per day) in the catchment area of the Harbor Area Treatment Scheme is treated by the CEPT process [10]. The objective here was to generate effluent that can be directly discharged to the sea. There are also several other case studies of CEPT large scale applications [11][12][13][14] where the objective was typically to reduce the load on the downstream process.
The carbon, suspended solids, and nutrient removal efficiencies of CEPT reported in the literature are given in Table 1. Most of these studies were conducted using a bench-scale setup. As can be seen from Table 1

Application and Efficiencies of CEPT Processes
Harleman and Murcott [9] provided CEPT application examples from Mexico City, Southern California, Hong Kong, Sao Paulo, and Rio de Janeiro. For instance, in Hong Kong, over 75% of sewage (~2.2 million m 3 per day) in the catchment area of the Harbor Area Treatment Scheme is treated by the CEPT process [10]. The objective here was to generate effluent that can be directly discharged to the sea. There are also several other case studies of CEPT large scale applications [11][12][13][14] where the objective was typically to reduce the load on the downstream process.
The carbon, suspended solids, and nutrient removal efficiencies of CEPT reported in the literature are given in Table 1. Most of these studies were conducted using a bench-scale setup. As can be seen from Table 1, CEPTs can remove between 43.1-95.6% of COD, 70.0-99.5% suspended solids, and 40.0-99.3% of phosphate depending on the characteristics of wastewater treated and type of coagulants and/or flocculants used. In some of the cases where very high COD removals were reported, due to the increased redirection of carbon to the CEPT sludge, the carbon going to the subsequent secondary treatment processes may be low, and the CEPT effluent may not meet the requirement for denitrification process that is required for nitrogen removal. As far as nutrient removal is concerned, despite phosphorus removal, the removal of nitrogen using CEPT is limited. Therefore, CEPT is often combined with other processes (such as shortcut nitrification-denitrification, ion exchange) if removal of nitrogen is required [12,[15][16][17].
Dolejs et al. [18] considered two process schemes to achieve energy neutrality or reduce energy consumption using CEPT in wastewater treatment facilities. First, a two-stage biological process, i.e., a high-rate primary treatment for COD capture (generally named A-stage) and biological nutrient removal systems as secondary treatment (B-stage). Second, innovative biological nitrogen removal schemes based on shortcut nitrogen removal via nitritation/denitritation or partial nitrification with an anammox process. Wan et al. [15] reported that the A-B processes that include CEPT are the most feasible options to improve energy recovery and minimize in-plant energy consumption. The COD flow and biogas production potential in typical conventional activated sludge process and A-B process with CEPT as A-stage and shortcut nitrification and denitrification as B-stage is shown in Figure 2. In addition to the nitrogen removal in the A-B process, notice the COD mass flow to the CHP (combined heat and power plant) (Figure 2a,b) indicate 33% of influent COD ultimately converted to biomethane in the conventional process compared to 43% for the A-B process.  CEPT can increase the dissolved heavy metal removal efficiencies of chromium, copper, nickel, and zinc from raw wastewater, preventing heavy metals from reaching downstream processes and the receiving waters [23]. It should be noted that the increased metal precipitation and accumulation in the CEPT sludge could potentially affect the subsequent sludge treatment and reuse processes. Therefore, disposal of CEPT sludge rich in harmful metals should be done carefully to avoid the adverse impact on the environment. CEPT is also an excellent option to remove dyes from wastewater. Riera-Torres et al. [46] demonstrated that a combination of membrane filtration and CEPT could provide almost 100% of dye removal. CEPT is also one of the most commonly used methods in tannery wastewater treatment [47]. Micropollutants can also be removed by CEPT [48][49][50]. A study conducted by Zhou et al. [50] showed that 65% of retinoids and 73% of endocrine-disrupting chemicals (EDCs) were removed from municipal wastewater after aluminum (Al)-based CEPT. In comparison, 80% of retinoids and 72% of EDCs were removed after iron (Fe) based CEPT. A medium-low removal efficiency (38-55%) of hydrophobic organic micropollutants (OMPs) was reported by Taboada-Santos et al. [51], where the removal is attributed to the sorbed fraction of the OMPs into total suspended solids and flocs formed.

Mechanisms of Coagulation and Flocculation
Coagulation and flocculation are the processes involved in CEPT. Coagulation refers to the process where particles are destabilized, and the aggregates (coagula) tend to be small and dense, whereas flocculation referrers to the process where aggregates (flocs) tend to be larger and more open in structure [52]. The main mechanisms in coagulation can be classified into (1) double layer compression, (2) adsorption and charge neutralization (3) entrapment of particles in the precipitate, and (4) interparticle bridging. These mechanisms are described in Table 2 [1,[53][54][55][56]. The classical method of colloid destabilization is double-layer compression [56]. For the case where a polymer and the adsorption site are of opposite signs, it is postulated that charge neutralization is the dominant mechanism. Chemical sorption and accumulation in the CEPT sludge is the mechanism for the removals of micropollutants [50]. The mechanisms of flocculation are (1) micro-flocculation (also known as perikinetic flocculation) in which particle aggregation is brought about by the random thermal motion of fluid molecules known as Brownian motion or movement and (2) macro-flocculation (also known as orthokinetic flocculation) in which particle aggregation is brought about by induced velocity gradients and differential settling [1]. Table 2. Mechanisms of coagulation.

Double-layer compression
The zeta potential represents the net charge of colloidal particles. Consequently, the higher the value of the zeta potential, the greater is the magnitude of the repulsive power between the particles and hence the more stable is the colloidal system. Coagulation increases the ionic strength of the wastewater, which has the effect of compressing the electrical double layer (EDL), which in turn reduces the zeta potential. If this layer is sufficiently compressed, then the van der Waals force will be predominant across the entire area of influence, so that the net force will be attractive and no energy barriers will exist. The net repulsive energy would become smaller or even would be eliminated, allowing the particles to approach each other and agglomerate ( Figure S1).

Adsorption and charge neutralization
The aqua metallic ions (Al(H 2 O) 6 3+ or Fe(H 2 O) 6 3+ )) formed during coagulation become part of the ionic cloud surrounding the colloid and, because they have a great affinity for surfaces, are adsorbed onto the surface of the colloid where they neutralize the surface charge. Once the surface charge has been neutralized, the ionic cloud dissipates, and the electrostatic potential disappears so that contact occurs freely.

Entrapment of particles in the precipitate (sweep coagulation)
When the coagulants alum [Al 2 (SO 4 ) 3 ] or ferric chloride (FeCl 3 ) are added in high enough concentration, they will react with hydroxides (OH − ) to form metal hydroxide precipitates, Al(OH) 3(s) or Fe(OH) 3(s) , respectively. The colloidal particles become entrapped in the precipitates either during the precipitate formation or just after.

Interparticle bridging
Bridging occurs when segments of a high-molecular-weight polymer adsorb on more than one particle, thereby linking the particles together. When a polymer molecule comes into contact with a colloidal particle, some of the reactive groups on the polymer adsorb on the particle surface and other portions extend into the solution. If a second particle with an open surface can adsorb the extended molecule, then the polymer will have formed an interparticle bridge.

Synthetic Coagulants
The commonly used synthetic coagulants fall into two general categories: those based on aluminum and those based on iron. Aluminum-based coagulants include aluminum sulfate (alum), aluminum chloride, sodium aluminate, aluminum chlorohydrate, and sodium aluminate. Iron coagulants include ferric sulfate, ferrous sulfate, chlorinated copperas, ferric chloride, polysilicate-iron (PSI). Alternative coagulants based on pre-hydrolyzed forms of aluminum (such as poly aluminum chloride (PACl)) and iron (poly ferric sulfate (PFS)) are more effective than the traditional additives [57]. The popularity of aluminum and iron coagulants arises not only from their effectiveness as coagulants but also from their ready availability and relatively low cost. The efficacy of these coagulants arises principally from their ability to form multi-charged poly-nuclear complexes in solution with enhanced adsorption characteristics [3]. Other chemicals used as coagulants include hydrated lime and magnesium carbonate. Moreover, Ecofloc CP [58], cement kiln dust [24], and fly ash-based coagulants [59] were also found successful for municipal wastewater CEPT applications. Due to their superior color removal even at small dosage and affectivity at wider pH range PACL, PFS, poly aluminum ferric chloride (PAFCl), and poly ferric chloride (PFCl) may be considered as better coagulants for the treatment of industrial textile wastewaters of wastewater [2].

Recycled Chemical Sludge
Some studies investigated methods of recovering coagulants from water treatment and CEPT sludges and use it for CEPT, which will probably make the CEPT process more cost-effective. There are different methods of coagulant recovery from water treatment residuals that include acidification, basification, ion exchanging, and membranes [44]. For example, in the acidification method, alum and iron salts may be recovered from sludge by adding sulfuric acid (Equation (1)).
where, Metal represents aluminum or iron. Recycling coagulants has both advantages and limitations. Xu et al. [60] have shown that the quality of treated wastewater by recovered and fresh coagulant is similar, and the effect of recovered coagulant might be better than that of fresh coagulants in some respects, such as turbidity removal. A similar study conducted by Xu et al. [44] verified the possibility of ferric coagulant recovery using acidification from CEPT sludge and its recycled product in CEPT to make the process more cost-effective, as well as reduce sludge volume. Chakraborty et al. [28] studied the reusability of recovered iron coagulant from primary municipal sludge. They showed that an annual saving in operating costs of a 1 MGD plant of USD 5052 could be achieved. A similar study conducted by Taheriyoun et al. [61] investigated the feasibility of chemical sludge as a coagulant aid in the coagulation-flocculation process of steel mill wastewater treatment. This study revealed that the use of CEPT sludge could reduce up to 60% of the coagulant and flocculent dose without adversely affecting the final turbidity. Considering the economic benefits due to chemical savings, the use of recovered coagulants is recommended. However, coagulant recovery is at its early stage of research, and cost-benefit analysis needs to be done to ensure sustainability and its application at a larger-scale. It should also be noted that in coagulant recovery processes such as acidification or basification, other metals may be solubilized along with the coagulant metal, and reuse of the recovered coagulant will recycle metals that should again be precipitated along with the coagulant [55].
As explained above, using recovered coagulants from sludge is one option to reduce the cost involved in chemicals added for CEPT and promote sustainability. The other option is using recovered iron or alum salts from wastes from some industrial processes. For example, titanium oxide (TiO 2 ) production plants generate a large amount of iron as iron sulfate or chloride salt as a waste. Based on global TiO 2 production data from 2007, around 1.3 million tonnes or 40% of the total iron contained in TiO 2 mineral resources is not recovered but is lost as waste [62]. Therefore, an innovative technology could be adopted to recover this iron and use it for CEPT, which would, in turn, maximize waste recovery.

Natural Coagulants
Natural coagulants are non-toxic, biodegradable, environment friendly, and have the ability to encapsulate [2]. Natural coagulants will not hinder biological treatment (if required) and may act as nutrients for the microorganisms. Moreover, organic coagulants, as opposed to inorganic ones, do not cause a significant increase in chloride and sulfate ion concentrations, which facilitates the use of treated wastewater in the water reuse systems, such as irrigation of crops [63]. Czerwionka et al. [63] studied the impact of organic coagulants on the efficiency of nutrient removal processes and verified that the organic coagulants did not have a significant impact on the values of unit processes rates associated with the biological removal of nitrogen and phosphorus in the activated sludge process. The organic coagulants used in this study are Cofloc, C29510, and Sedifloc 575 [63]. In another study, Luo et al. [64] assessed and verified the capability of CEPT for the treatment of printing and dyeing wastewater (PDW) using three different coagulants (ferrous sulfate, aluminum sulfate, and polymeric iron sulfate). One of the novel findings of this study is that the microbial community of PDW from the industrial park after CEPT was similar to the raw wastewater. Moreover, natural coagulants do not induce a change in the physicochemical properties of treated water [7].
Natural coagulants could be plant-based, animal-based, or made from microbes. Plant-based coagulants (especially, Moringa oleifera) have shown significant promise in treating the wastewater emanating from different types of industries [65]. Cheng et al. [33] investigated the feasibility of guar gam for sustainable landfill leachate treatment and reported that 22.6% of COD removal could be achieved at guar gum dosage of 44.4 mg/L. The removal of COD efficiency reported by Cheng et al. [33] is less than studies conducted using synthetic/inorganic coagulants for leachate treatment. For example, Ghafari et al. [31] reported 62.8 % and 43.1%. COD removal was using Alum (dosage = 2000 mg/L) and PACl (dosage = 9500 mg/L), respectively ( Table 1). The reason for the lower removal efficiency of the natural coagulant could be attributed to the lower dosage of the coagulant. To obtain an equivalent COD removal efficiency as conventional coagulants (inorganic coagulants), a larger dose of natural coagulants is required due to its weaker coagulation capability [66].

Coagulant Selection Procedure
Although there are many types of coagulants depicted above, the most effective coagulant for particular wastewater still largely depends on the outcome of laboratory jar testing [31] and the optimization of each plant is required to give maximum efficiency in the selection of both chemical and dose [67]. Feo et al. [58] defined a simple multi-criteria procedure to select the best coagulant as well as the relative dose based on the adoption of the following five criteria: COD percentage removal, sludge volume after two hours, coagulant dose, coagulant cost and pH percentage variation. The multi-criteria procedure defined by Feo et al. [58] can be summarized in the following six steps (considering five criteria for evaluation):

1.
Construct a complete alternatives matrix (CAM), composed of five columns (the criteria) and as many rows as the number of alternatives.

2.
Eliminate from the CAM the rows relating to the alternatives with a COD percentage removal less than 40%, thus obtaining a simplified alternative matrix (SAM).

3.
Normalize the SAM. The normalization is obtained by dividing each value in the first column by the maximum value of the same column and then dividing the minimum value in the other columns by the considered value.

4.
The fourth step is to attribute a weight to each criterion, distributing the value 10 among the 5 criteria adopted. An "equilibrate" weight distribution is obtained by multiplying each value by 2. This corresponds to giving a 20% relevance to each criterion in the choice of the best coagulant and dose.

5.
Calculate for each alternative an average index (AI) obtained by taking the average of the 5 weighted values for each alternative. 6.
List the alternatives in decreasing order in terms of AI. The first coagulant and the relative dose will be the best choice.
Based on the procedure described by Feo et al. [58], Dong et al. [26] used equilibrate weight distribution and therefore assigned an equal weight of 2 to each criterion (removals of tCOD, sCOD, TP, and TSS), and determined the corresponding index for each polymer and its impact on each parameter. Averages of the four weighted values were then compared (Tables S1 and S2), and the polymer with the highest index (PACL) was selected as the optimal polymer.
Although the cost of the coagulants is one of the five criteria defined by Feo et al. [58], the procedure illustrated by Dong et al. [26] did not include cost, which needs to be included as one criterion. The cost of chemicals used in CEPT should be given due consideration when comparing coagulants. The cost of commonly used coagulants and flocculants are depicted in Table 3. A similar procedure could be adopted for the selection of the coagulant. Furthermore, modeling of the CEPT process is required to evaluate the performance of the process in large-scale applications. Since influent quality is affected by several site-specific conditions, the preliminary collection of site-specific data for COD fractionation is always beneficial [68]. Moreover, the overall cost-effectiveness and environmental impact of coagulants should be considered in the evaluation of coagulants. It should also be noted that the addition of chemical costs will increase the capital and operating costs of wastewater treatment. However, the cost associated with chemical addition could be compensated by the benefit of CEPT that includes energy recovery from the CEPT sludge and a reduction in the aeration energy required for secondary treatment due to the increased removal of carbon. Diamantis [69] studied the cost and benefits of CEPT under controlled laboratory conditions and produced a conceptual design for a community of 2000 inhabitant equivalents. According to Diamantis [69], the capital and operational costs of CEPT were 0.11 and 0.09 EUR/m 3 , respectively, and the biogas generated is used for digester heating and the overall process is energy self-sufficient. Table 3. Cost of commonly used coagulants and flocculants in CEPT.

Flocculants Used in CEPT
Flocculants used in CEPT are mainly synthetic polymers that can be classified into three types, depending on their charge as cationic (positively charged), anionic (negatively charged) and nonionic (uncharged). There are also naturally occurring flocculants (starch derivatives, guar gums, tannins, alginates, etc.). When the polymer is used in combination with a conventional coagulant, the aim is to not only achieve better floc characteristics in terms of size, strength, settleability and filterability but also earn considerable savings on coagulant dose for the same removal performance [71]. Examples of polymers, which are commonly used in CEPT, are listed in Table 4. These polymers have different chemical structures, vast ranges of molecular weight, and significant differences in charge density. For nonionic polymers, the attraction between particles is entirely the result of the "bridging" effect. However, for cationic and anionic polymers, there is also the possibility that charge neutralization can play a role. Both of these effects may operate simultaneously [55]. Among the polymers listed in Table 4, PACl and PAM are widely used in CEPT. Dong et al. [26] compared the removal efficiencies of seven different cationic, anionic and nonionic flocculants and found that PACl is the best performing flocculant for municipal wastewater. Using natural polymers as flocculants is a preferable option for CEPT. In general, the advantages of natural polymers are that they are virtually toxic-free, biodegradable in the environment and the raw products are often locally available, whereas industrialized coagulants may not be [3]. Another alternative is the use of biological flocculants (biopolymers/bioflocculants) derived from bacteria and fungi, which could be environmentally friendly [72]. Numerous works have demonstrated that chitosan and its derivatives (in particular grafted biopolymers) can be a potential substitute for metallic salts and synthetic polyelectrolytes in the treatment of wastewater for the removal of both particulate and dissolved substances [40,57,73]. Therefore, chitosan is one of the most promising natural flocculants. Moreover, chitosan can form complexes with metallic ions [74]. Some studies explored the combined application of natural and synthetic chemicals in CEPT. Bhuptawat et al. [22] studied the use of a natural coagulant (water extract of Moringa oleifera seed) in combination with alum (Al 2 (SO 4 ) 3 ·18H 2 O) for municipal wastewater treatment. According to this study, with the coagulant combination of 100 mg/L of M. oleifera with 10 mg/L alum, a maximum overall removal of 64% of COD was achieved (Table 1).

Impact of CEPT on Different Wastewater Fractions
Wastewater characteristics and corresponding fractions vary from place to place. For example, Pasztor et al. [75] collected and summarized the data of wastewater COD fractions from several countries and reported that the ratio of COD components in raw wastewater is very different. The average composition is as follows: Inert particulate = 17.1%, slowly biodegradable = 57.9%, inert soluble = 7.8% and readily biodegradable = 17.5% [75]. In addition to particulate COD and P fractions, CEPT can remove colloidal fractions (soluble and slowly degradable) of wastewater. The colloidal particles found in wastewater typically have a net negative surface charge. The size of colloids (about 0.001 to 1 mm) is such that the attractive body forces between particles are considerably less than the repelling forces of the electrical charge [1]. Under these stable conditions, Brownian motion (i.e., random movement) keeps the particles in suspension. The coagulation process in CEPT, as previously described in Table 2, destabilizes the colloidal particles so that particle growth can occur as a result of particle collisions and finally settle.
To better understand the impact of CEPT on removal efficiency assessment of different COD and P components is crucial. Although extensive research has been carried out on CEPT in both bench-and full-scale operations, no single study exists which adequately covers the impact of CEPT on different COD and P fractions. We evaluated the CEPT process considering the different COD and phosphorus fractions and indicated the fractions that can potentially be removed from wastewater (Figures 3 and 4). Procedures and methods described by the Water Environment Research Foundation (WERF) [76] and standard methods for the examination of water and wastewater [77] can be used for determining the COD and P fractions, respectively.

Impact of CEPT on Different Wastewater Fractions
Wastewater characteristics and corresponding fractions vary from place to place. For example, Pasztor et al. [75] collected and summarized the data of wastewater COD fractions from several countries and reported that the ratio of COD components in raw wastewater is very different. The average composition is as follows: Inert particulate = 17.1%, slowly biodegradable = 57.9%, inert soluble = 7.8% and readily biodegradable = 17.5% [75]. In addition to particulate COD and P fractions, CEPT can remove colloidal fractions (soluble and slowly degradable) of wastewater. The colloidal particles found in wastewater typically have a net negative surface charge. The size of colloids (about 0.001 to 1 mm) is such that the attractive body forces between particles are considerably less than the repelling forces of the electrical charge [1]. Under these stable conditions, Brownian motion (i.e., random movement) keeps the particles in suspension. The coagulation process in CEPT, as previously described in Table 2, destabilizes the colloidal particles so that particle growth can occur as a result of particle collisions and finally settle.
To better understand the impact of CEPT on removal efficiency assessment of different COD and P components is crucial. Although extensive research has been carried out on CEPT in both benchand full-scale operations, no single study exists which adequately covers the impact of CEPT on different COD and P fractions. We evaluated the CEPT process considering the different COD and phosphorus fractions and indicated the fractions that can potentially be removed from wastewater (Figures 3 and 4). Procedures and methods described by the Water Environment Research Foundation (WERF) [76] and standard methods for the examination of water and wastewater [77] can be used for determining the COD and P fractions, respectively.   Colloidal COD for conventional primary effluent wastewaters ranges from 29 to 36% of the total COD [80]. In CEPT, due to the coagulation and flocculation process, the colloidal fraction is significantly removed, resulting in very low to no colloidal COD in the effluent. It should be noted that a well-operated CEPT is capable of almost completely removing colloidal matter [81]. Therefore, Colloidal COD for conventional primary effluent wastewaters ranges from 29 to 36% of the total COD [80]. In CEPT, due to the coagulation and flocculation process, the colloidal fraction is significantly removed, resulting in very low to no colloidal COD in the effluent. It should be noted that a well-operated CEPT is capable of almost completely removing colloidal matter [81]. Therefore, assuming a typical total COD concentration of conventional primary effluent (290 mg/L) and considering only the colloidal COD removal that can be achieved by CEPT, the total COD concentration in the CEPT effluent could range from 186 to 206 mg/L.

CEPT Sludge Treatment, Recovery, and Disposal
The sludge production in CEPT is higher than the sludge produced from conventional primary treatment due to the chemicals added. The amount of sludge produced by CEPT can be estimated by Equation (2) [82]. The energy production from a CEPT sludge in an anaerobic process can be calculated from a simple mathematical correlation (Equation (3)) as a function of the process efficiency (ηAD) and the sludge concentration fed into the anaerobic digestor [83]. Equation (2) was derived by considering typical values from a municipal WWTP and a calorific value of 11 kW h/N m 3 CH 4 for methane. It should be noted that the energy production might be affected by the increase in inorganic precipitates in the sludge, which could lower the efficiency of biogas production [1].
where SP = sludge production (g SS/m 3 ) SS in = suspended solids concentration in influent (g SS/m 3 ) SS out = suspended solids concentration in effluent (g SS/m 3 ) K prec = sludge production coefficient (g SS/g Metal), typically 4-5 for Fe and 6-7 for Al D = Dossage of coagulant (g Metal/m 3 ) where E = Energy production (KW h/m 3 sludge ) c = Sludge concentration (Kg solids/m 3 ) η AD = Anaerobic digestion efficiency (typically, 40%-50% in full-scale digesters) The organic carbon in CEPT sludge could be recovered by hydrolysis and acidogenesis, and the VFAs-rich fermentation liquor can be dosed into wastewater to make up the carbon deficit for denitrification [84]. Some studies conducted on CEPT sludge investigated the degradability of CEPT and the production of biogas from CEPT sludge using anaerobic digestion. The organic carbon transferred to CEPT sludge could increase anaerobic digester gas production or promote autogenous combustion in sludge incineration. On the other hand, CEPT sludge, especially using Al-based coagulants, features a rather low biodegradability and treatability due to the agglomeration of solid matters by the coagulant [25]. Some researchers now argue that some polymers used in CEPT have an adverse effect on the quality of CEPT sludge. Lin et al. [85], for example, argues that PACl, which is one of the best coagulants/flocculants for CEPT, has an inhibitory effect on organic hydrolysis of the CEPT sludge. Therefore, further studies on CEPT, which use such type of polymers, need to assess the inhibitory and/or toxicity effect of the polymers on the downstream treatment processes.
CEPT sludge has good filterability and dewaterability compared to conventional primary treatment sludge, which will reduce the cost of drying or dewatering processes [20,86]. It should also be noted that the higher phosphate removal efficiency of CEPT results in a significant amount of phosphate in the sludge. Therefore, there is a need for a P recovery process that separates it from other waste components, so that it can be recycled as a fertilizer or an ingredient in other valuable phosphorus products [87].
COD and P fractions removed can be recovered from the CEPT sludge using different methods that include anaerobic digestion and incineration to recover energy from COD; and physical, thermal, chemical, and biological technologies to recover P [14,79]. These available technologies recover P from wet sludge or sludge ash through either accumulation, release, or extraction [88]. However, some studies pointed out that the current P recovery technologies implemented in pilot or large-scale applications that include wet-chemical extraction/leaching, wet-oxidation, precipitation, and thermochemical treatment are associated with comparatively high emissions and energy demand making it difficult to present an ideal recovery technology [89,90].

Comparison of CEPT with Other Alternative Methods
As described in the introduction section, CEPT is a chemical process where chemicals are added for the up-concentration of organic matter. However, there are also other processes that could be employed for the up-concentration of organics and capturing them onto sludge. These processes can be considered as an alternative to CEPT and include (1) physical processes (dynamic sand filtration and membrane filtration), and (2) biological processes (adsorption/bio-oxidation process and contact stabilization) [91]. A detailed review of these alternative processes has previously been provided by Sancho et al. [91] and Guven et al. [92]. Guven et al. [92] discussed these processes and provided their strengths and constraints. Sancho et al. [91] suggested that a combination of both physical and biological separation processes is a promising process for carbon redirection.
Some studies proposed high-rate contact stabilization (HiCS) as a promising primary treatment technology with which to maximize the redirection of organics to sludge for subsequent energy recovery [93][94][95]. The HiCS utilizes a feast-famine cycle to select for bioflocculation, intracellular storage, or both. Return sludge is aerated in a stabilization phase (famine) before it is brought into contact with wastewater under anaerobic conditions (feast). Meerburg et al. [94] showed that HiCS, as a primary wastewater treatment technology, was able to recover up to 55% of incoming organic matter as sludge. Based on a pilot-scale study, Dolejs et al. [18] reported a COD removal of 76.6% using bioflocculation. Dolejs et al. [18] argue that bioflocculation-enhanced primary treatment has several benefits over CEPT or conventional primary treatment. In summary, a holistic assessment considering the advantages and disadvantages of these alternatives in comparison to CEPT should be made to decide the best method for the up-concentration of organic matter.

Environmental Impacts
Most CEPT studies focused on performance evaluation of CEPT and optimization of coagulants and flocculants dosages. These studies have based their criteria for optimization and selection on carbon, phosphorus, and suspended solids removal rates and far too little attention has been paid to the impact of CEPT on the environment. Achieving lower residual coagulant or flocculants concentration in the effluent and minimizing adverse effects in the downstream biological processes should be considered and evaluated in CEPT applications to avoid environmental impact. It should be noted that coagulant recovery processes might have the potential to reduce the environmental impacts of water and wastewater coagulation processes [96]. An alternative method where chemicals are not needed is using bio-flocculation and adsorption, as the sludge itself can be used as a flocculent to enhance the treatment results [97]. The additional cost of chemicals is eliminated, and at the same time, the impact of chemicals in downstream processes is avoided.
The development of a holistic approach for the environmental impact of CEPT is complex, and appropriate metrics such as footprint, required wastewater quality of the primary effluent, plant capacity, greenhouse emission, etc., should all simultaneously be considered [70]. The available assessment methods presented by Maktabifard et al. [5], could help in analyzing the impact of CEPT on the environment. These assessment methods include economic Efficiency analysis (EEA), carbon footprint analysis (CFA), life cycle analysis (LCA), data envelopment analysis (DEA), and plant-wide modeling.

Conclusions
CEPT is a successful and preeminent method for carbon redirection and nutrient removal and can be considered as a promising method to recover resources from wastewater. The overall cost-effectiveness and environmental impact of coagulants should be considered in the evaluation of coagulants. To satisfy many of the principles of sustainability, the recovery of carbon and phosphorus from the CEPT sludge and reuse of coagulants are essential. The limitation in CEPT application studies conducted so far lies in the fact that the environmental impact of coagulants and flocculants is not addressed. These chemicals used in CEPT may be toxic and affect the biodegradability of CEPT sludge and/or the performance of downstream biological treatment processes. Moreover, the characteristics of final effluent, which is discharged to receiving water bodies, may be affected. Therefore, coagulants and flocculants selection should also address the minimization of environmental impact. The use of natural coagulants and environmentally friendly flocculants are encouraged to avoid toxic effects and promote sustainability.
Supplementary Materials: The following are available online at http://www.mdpi.com/2071-1050/12/15/5928/s1, Figure S1: Attractive and repulsive forces that result when two particles are brought together, Table S1: Comparison of different polymers, Table S2: Alternative matrix of polymers using equilibrate weight distribution.
Author Contributions: W.A.S. conceived and prepared the draft manuscript, M.D. reviewed, edited, and contributed to the final paper. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.