Review Of Synthetic Human Faeces And Faecal Sludge For Sanitation And .

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Water Research 132 (2018) 222e240Contents lists available at ScienceDirectWater Researchjournal homepage: www.elsevier.com/locate/watresReviewReview of synthetic human faeces and faecal sludge for sanitation andwastewater researchRoni Penn a, *, Barbara J. Ward a, Linda Strande a, Max Maurer a, babEawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, SwitzerlandInstitute of Civil, Environmental and Geomatic Engineering, ETH Zürich, 8093 Zurich, Switzerlanda r t i c l e i n f oa b s t r a c tArticle history:Received 17 July 2017Received in revised form22 December 2017Accepted 23 December 2017Available online 30 December 2017Investigations involving human faeces and faecal sludge are of great importance for urban sanitation,such as operation and maintenance of sewer systems, or implementation of faecal sludge management.However, working with real faecal matter is difficult as it not only involves working with a pathogenic,malodorous material but also individual faeces and faecal sludge samples are highly variable, making itdifficult to execute repeatable experiments. Synthetic faeces and faecal sludge can provide consistentlyreproducible substrate and alleviate these challenges. A critical literature review of simulants developedfor various wastewater and faecal sludge related research is provided. Most individual studies sought todevelop a simulant representative of specific physical, chemical, or thermal properties depending ontheir research objectives. Based on the review, a suitable simulant can be chosen and used or furtherdeveloped according to the research needs. As an example, the authors present such a modification forthe development of a simulant that can be used for investigating the motion (movement, settling andsedimentation) of faeces and their physical and biological disintegration in sewers and in on-site sanitation systems. 2018 Elsevier Ltd. All rights reserved.Keywords:Fecal sludgeFecal sludge simulantFecesFeces simulantsOnsite wastewater treatmentSewersContents1.2.3.4.5.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223Characteristics of human faeces and faecal sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2232.1.Faeces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2232.2.Faecal sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224Synthetic faeces and faecal sludge found in the literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2243.1.Physical parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2243.1.1.Faeces simulants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2253.1.2.Faecal sludge simulants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2263.2.Chemical, biological, and thermal parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2283.2.1.Faeces simulants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2283.2.2.Faecal sludge simulants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2304.1.Making use of Table 8 for development of a new simulant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2334.1.1.New faeces simulant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2334.1.2.New faecal sludge simulant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234Development of new simulant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235Material and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235Chemicals and materials used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235Measurement methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235* Corresponding author.E-mail address: roni.penn@eawag.ch (R. 0043-1354/ 2018 Elsevier Ltd. All rights reserved.

R. Penn et al. / Water Research 132 (2018) 222e240223Base synthetic faeces recipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236Physical structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236Chemical composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238Recommendation for recipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238Recommended storage practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238Synthetic faeces production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2391. IntroductionInvestigations involving human faeces are of great importancein many fields of research, such as medicine (Lewis and Heaton,1997; Bekkali et al., 2009), sanitary product development (such asdiapers, toilets etc.) (Stern and Holtman, 1987; Palumbo andD'acchioli, 2001), operation and maintenance of sewer systems(Butler et al., 2003; Penn et al., 2017), and implementation of faecalsludge collection and treatment for onsite sanitation systems n et al., 2015).(Wignarajah et al., 2006; Bassan et al., 2014; ColoDevelopment of synthetic faeces and faecal sludge is a challengingtask due to their high variability depending on diet, lifestyle andgeographical location (Rose et al., 2015). In this paper, we focus onsynthetic faeces and faecal sludge developed for sanitationresearch, hence resembling human stool and faecal sludge in specific physical and chemical properties. The high variability of faeces(Rose et al., 2015) and faecal sludge collected from onsite systems(Strande et al., 2014) makes it difficult to obtain consistent samplesand therefore execute repeatable experiments. Moreover, due tothe potentially pathogenic content of human excreta, working withreal faecal matter involves special safety precautions. Working withsynthetic faecal matter can alleviate these challenges.Faeces and faecal sludge are different. Faecal sludge is the faecalwaste stored within onsite sanitation technologies. In addition tofaeces it includes everything that goes into the toilet, for example,urine, flush water, greywater, anal cleansing materials and municipal solid waste (Strande et al., 2014). Faecal sludge differs significantly from fresh faeces alone; it is typically much more dilute dueto the addition of liquids. Additionally, its characteristics are highlyvariable due to differences in storage duration, storage temperatureand storage technology, and can range from fresh, to partiallydegraded, to completely stabilized (Strande et al., 2014). Syntheticfaeces have been developed to address many sanitation relatedresearch questions. Most of the developed simulants mimic specificphysical, chemical or thermal characteristics of human faecesimportant to the research objectives for which they are developed.Physical properties such as shape, size, density and rheology are ofimportance for simulating phenomena such as faeces settling,transport in sewer pipes, dewatering, viscous heating for pathogendestruction, and physical disintegration (e.g., Butler et al., 2003;Veritec Consulting Inc. & Koeller and Company, 2010; Podichettyet al., 2014). Chemical properties including chemical and biological oxygen demand, nutrient concentration, pH and conductivityare of importance for simulating biological disintegration, treatment of faeces and biogas production (e.g., Kaba et al., 1989;Wignarajah et al., 2006; Miller et al., 2015). Heating propertiesand elemental composition (C,H,N,O) are of importance for analysing energy recovery and for using the faeces for soil amendment ne.g., biochar or compost production (e.g., Ward et al., 2014; Coloet al., 2015; Onabanjo et al., 2016a). Studies on the fate of faecesin sewers and in onsite sanitation systems include their movement,settling and physical disintegration together with biochemicaldisintegration. For these kinds of investigations the simulant isrequired to obtain a combination of chemical, biological andphysical properties. Such a faeces simulant is still missing in theliterature.Three distinct recipes for synthetic faecal sludge have been reported in the literature. Their intended purposes include research n et al., 2015) and pitinto anaerobic digestion (Zuma, 2013; Cololatrine emptying (Radford et al., 2015). Together with the syntheticfaeces recipes presented in this review, they could be used as a basisfor the development of improved faecal sludge simulants in thefuture.In this article, we provide a critical literature review of syntheticfaeces and faecal sludge used for human waste related research.Based on this overview we present a modified simulant recipe thatis applicable to be used for studying the fate of faeces in sewers andin onsite sanitation systems. A series of experimental resultsshowing how these properties can be selectively manipulated bymaking changes in the recipe and an explicit preparation procedurecan be found in the appendix of this paper.2. Characteristics of human faeces and faecal sludge2.1. FaecesFaecal solids are composed of proteins, fats, fibre, bacterialbiomass, inorganic materials and carbohydrates. Their chemicaland physical characteristics vary widely depending on person'shealth and diet, as presented in Table 1. The average number ofstools produced by adults is one per day (Cibae Geigy, 1977). Themedian daily wet mass of faeces produced per person is 128 g (Roseet al., 2015), which falls within the reported full range of 35e796 greported by Ciba e Geigy (1977) and Rose et al. (2015). Wyman et al.(1978) compared average stool sizes of 20 people (average of 10samples from each individual). They identified that 250 g/stool and111.3 g/stool were the maximum of these averaged weights of themen and women participants, respectively, in the study. In theirreview of faeces characteristics Rose et al. (2015) further report thatlive and dead bacteria comprise between 25 and 54% of the dryweight of faeces. The median water content in faeces is 75%, with arange of 63e86% across mean values of studies. Variations in watercontent and faecal mass are attributed to differences in fibre intake,as non-degradable fibre absorbs more water in the colon anddegradable fibre stimulates growth of bacterial biomass (Eastwood,1973; Garrow et al., 1993; Reddy et al., 1998). Rose et al. (2015)report that volatile solids comprise 92% of the total solids (TS)fraction of faeces. Faeces pH ranges between 5.3 and 7.5, withbiological oxygen demand (BOD) between 14 and 33.5 g/cap/dayand chemical oxygen demand (COD) between 46 and 96 g/cap/day(Rose et al., 2015).Faeces are also highly variable in their physical structure. Thisvariability can be characterized by the “Bristol Stool Form Scale”introduced by Lewis and Heaton (1997) for assessing intestinal

224R. Penn et al. / Water Research 132 (2018) 222e240Table 1Chemical and physical properties of faeces found in the literature.Chemical propertiesPhysical propertiesaRangeRangeamount/cap/dOther unitsWet massWater contentProteinFibreCarbohydratesFatsBacteria content35e796 ga,fBODCODTNVSpHeCalorific value14e33.5 gf46e96 gf0.9e4 gfShapeViscosityDensity'cMedian63e86 wt%2e25 wt% of solids weight (þ50% of bacterial biomass)f0.5e24.8 gf4e24 g f1.9e6.4 g6 g/cap/d f9 g/cap/d f4.1 g/cap/d25 wt% of solids weight f8.7e16 wt% of solids weight f25e54 wt% of solids weight f100e2200 1012 cells/kg b5e7% wt% of solids weightf92 wt% of TSf5.3e7.5ff1.8 g/cap/df6.6f, 7.15 (average)b0.55 MJ/cap/df0.21e1.45 MJfType 1 (hard lumps) e type 7 (watery diarrhoea)3500e5500 cPs 1 g/ml for 10e15% of healthy humans a128 g/cap/df75 wt%fe3.6 (average)f1.06e1.09 (average)b,dLevitt and Duane, 1972 ; bCiba-Geigy, 1977; cYeo and Welchel 1994b; dBrown et al., 1996c; eLewis and Heaton 1997d; fRose et al., 2015e.transit rate. The scale categorizes stools into one of seven types,ranging from type 1 (hard lumps) to type 7 (watery diarrhoea).Types 3 and 4 (“hard, lumpy sausage” and “loose, smooth snake”)are classified as normal stool forms. Onabanjo et al. (2016a) identified the moisture content of each stool classification ranging from 50% (type 1) to 80% (type 7). The Bristol Scale has been used toassess stool form in the study of gastrointestinal disorders (e.g.,Garsed et al., 2014; Nolan et al., 2015). Woolley et al. (2014a)measured the rheological properties of fresh human faeces. Theyshowed that with increasing shear rate the apparent viscositymeasurements of the samples decreased. For any given shear rate,higher apparent viscosities were associated with lower moisturecontents. Viscosity measurements of runny to solid faeces werefound to be in the ranges of 3500e5500 cP (Yeo and Welchel, 1994).According to the US National Bureau of Standards (NBS) faeces arecharacterized by density of 1.06 g/ml (Brown et al., 1996). 10e15% ofhealthy humans produce stool that floats (has a density less than1 g/ml) due to trapped gas in the faeces (Levitt and Duane, 1972).2.2. Faecal sludgeFaecal sludge originates from onsite sanitation technologies, andhas not been transported through a sewer. It is raw or partiallydigested, a slurry or semisolid, and results from the collection,storage or treatment of combinations of excreta and blackwater,with or without greywater (Strande et al., 2014). Blackwater isdefined as wastewater generated by the toilet, and includes excretaas well as flush water and anal cleansing water and/or dry analcleansing materials (Tilley et al., 2014). Greywater contain all otherdomestic wastewater flows including bathing, washing, laundryand kitchen (Gross et al., 2015).Typical quantities and qualities of faecal sludge are difficult todetermine due the variety of onsite sanitation technologies in use,such as pit latrines, septic tanks, aqua privies, and dry toilets. Theyfurther depend on the design and construction of the sanitationtechnology, how the technology is used, how the faecal sludge iscollected, and the frequency of collection (Strande et al., 2014).Recent findings have indicated that faecal sludge characteristics arecorrelated to the containment technology, but that there is nodiscernible difference between faecal sludge from public toilets andhouseholds (Strande et al., in preparation). Lack of standardizedmethods for the characterization of faecal sludge further contributeto the variability in the measured parameters (Velkushanova et al.,in preparation).The important parameters to be considered for faecal sludgecharacteristics are similar to those of faeces, they are presented inTable 2.3. Synthetic faeces and faecal sludge found in the literatureAppropriate simulants for faeces and faecal sludge should beable to reflect the range of physical, chemical, biological and thermal characteristics relevant for the research objective. This specifically includes: physical characteristics e.g. represented by the Bristol stool scale(for faeces simulants), shapability into the characteristic faeces cylinder, and can bemade to float or sink (for faeces simulants), viscosity and dewatering properties, chemical and biological properties including COD, BOD, TN, pH,EC, TS, VS, elemental composition, biogas potential, thermal properties, such as calorific value and ash content, ability to physicality disintegrate with a resulting aqueous suspension having similar chemical properties as real disintegratedfaeces (for faeces simulants) and biologically degrade in arepresentative way (for faecal sludge simulants).This wide variety of faecal and faecal sludge properties pose asubstantial challenge for creating a universal synthetic replacement. Indeed, such an optimal simulant has not yet been developed. Simulants found in the literature were developed toreproduce specific characteristics of human stool or faecal sludge,depending on the research objectives, with varying degrees ofsuccess. All developments were successful in producing a simulantthat is safe to use and does not represent any biohazard.3.1. Physical parametersThe following simulants are designed to reflect specific physicalproperties of human faeces and faecal sludge such as shape,rheology or density. As faeces are distinct from faecal sludge wediscuss each type of simulant separately.

R. Penn et al. / Water Research 132 (2018) 222e240225Table 2Physical and chemical properties of human faeces and faecal sludge compared with simulants.PropertiesFaecesSynthetic faecesShapeFrom “hard lump” to “watery diarrhoea”, “hardlumpsausage” and “loose smooth snake” are normalformscCylinderiLength (cm)Diameter (cm)Volume (ml)Density (kg/l)Viscosity (cP)Dewatering rate (g/m2/min)Shear strength (Pa)CODtotalFaecal sludgeSynthetic faecal sludge1e2.2ag8.9 10 1 6 109-ac11 (% of TS in the dewateredcake)ao,ap 1760ag7000e106,000 mg/lx,z0.8e2.2agNF*8e10i,u2.5e3.4i,u90-169 (for women)82-196 (for h,ab350-400 (for regular stool, very high for runnyfaeces)f0.6e1.5% of TSq,af,ahCODsolubleBODTNN-NH3 (% of Ntotal)C/NpHEC (mS/cm)TS (%)VS0.38% of TS ad14e33.5 g/cap/d ah2-7% of TS q,t,af,ah 7af5e16af4.6e8.4 e,ad,af,ahC (% of TS)H (% of TS)N (% of TS)O (% of TS)Fe (mg/kgTS)ZnNiCo (mg/kgTS)Mn (mg/kgTS)Mo (mg/kgTS)Cu (mg/kgTS)B (mg/kgTS)S44e55p,ae,ak,al7.0e7.6ae,ak,al1.1e18 60,256mg/kgTS1016e34,615mg/kgTS e254e3,846e46,857e236,539 e1148e12,180 e24,889e125,641 e14e37r,af,ah80-92% of TSa,d,e,af,50e400h1.3% of TSad,af2.8% of TS ad,af 3.02af17.3af5.3ad,af5.7ad18.4p,af86.8e88.5% of TSah,ak600e40,000 mg/l m,z50e1500 mg/lu,ab6.7e8.5s,z2.2e14.6am0.5e40am,w7000e52,000 mg/lx,zad,af,ak0.5e1.6% of TS43.4e47.3 k,aj,ak,af,al6.2e7.2 k,aj,ak,af,al2.1e7.2 k,aj,ak,af,al30e42 k,aj,ak,af,al59,950 ad46,210 mg/kgTS ad1289 ad642 ad6251 ad1555 ad5654 ad3524 ad0.06e0.19% of TSee,ae4.5 (% of TS in the dewateredcake)ao,ap9-10,000ag73ad12,500e72,800 mg/ly,ad1000e48,300 mg/ly,ad880e7200 1600e1800 mg/ly27.8e28.8an4.2an3.0e3.2an646-918 ppman24-30 ppman114-216 ppman388-1300 mg/lyk,v,afP (% of TS)Calorific value (MJ/kg)Ash (% of TS)Biogas yield0.39e4.93 e17.2e25.1b,e,j,l,ae,ah,ak9.7e14.6ae,ak0.16e0.53 NLbiogas/gCOD*a,e,o0.28 0.44 NLbiogas/gCOD*adAverage methane (% vol)Sulphatesoluble (mg/l)Total protein3.2e16.2 g/cap/dahProteinsoluble (mg/l)Total carbohydrates (mg/ 4-24 g/cap/dahl)Lipids0.09e0.16 g/gTS ah4.2 g/deTotal fiber (g/gTS)0.25 ahHemicellulose (g/gTS)Cellulose (g/gTS)Lignin (g/gTS)63ad0.24 NLbiogas/gVS y0.12e0.37 NLbiogas/gCOD*ad38e60ad88e392y2874e8835 mg/ly497e1,723y660e3,812y0.03e0.3 ll 1943; bLovelady and Stork 1970; cLevitt and Duane, 1972; dFry 1973; eCiba-Geigy, 1977; fWyman et al., 1978; gMeher et al., 1994; hYeo and Welchel 1994; iBrown et al., nsson et al., 2005;1996; jGirovich 1996; kTennakoon et al., 1996; lSpellman, 1997; mHeinss et al., 1999; nKoottatep et al., 2001; oPark et al., 2001; pEawag, 2002; qJorWignarajah et al., 2006; sHenze et al., 2008; tBarman et al., 2009; uVeritec Consulting Inc. & Koeller and Company, 2010; vSerio et al., 2012; wStill and Foxon 2012; xBassanyzaaabacadae n et al., 2015; Monhol and Martins, 2015;et al., 2013; Zuma 2013; Appiah-Effah et al., 2014; Muspratt et al., 2014; Podichetty et al., 2014; Strande et al., 2014; ColoafMiller et al., 2015; agRadford et al., 2015; ahRose et al., 2015; aiYerm an et al., 2015; ajIlango and Lefebvre 2016; akOnabanjo et al., 2016a; alOnabanjo et al., 2016b; amGold et al.,2017a; anGold et al., 2017b, aoWard et al., 2017a; apWard et al., 2017b.*NLbiogas/gCOD e normal liter (volume of gas at 273 K and 1 atm) of biogas to gCOD added.3.1.1. Faeces simulantsButler et al. (2003) prepared artificial faeces for laboratoryinvestigation of gross solids movement in sewers (referred to hereas simulant #1). Solids were represented with plastic cylinderswith a diameter of 3.4 cm, length of 8 cm and density of 1.06 g/ml,following the US NBS solid (Swaffield and Galowin, 1992). Pennet al., (Submitted) implemented similar solids for examining theirmovement in real sewers. Two techniques for tracking the grosssolids were developed; using light sticks tracked by computerizedlight detector and RFID (radio frequency identification) basedtracking. They further analysed the effect of reduced sewer flows onthe movement of the solids (Penn et al., 2017).Maximum Performance (Map) in the USA (Veritec ConsultingInc. & Koeller and Company, 2010) developed a media for testing

226R. Penn et al. / Water Research 132 (2018) 222e240toilet flush performance (simulant #2). In a ‘Toilet Fixture Performance Testing Protocol’, they define a test media (i.e., syntheticfaeces) to comprise the following: “one or more 50 4 g test specimen consisting of one of the following (i) soybean paste contained inlatex casing (cased media), tied at each end forming a ‘sausage’ or (ii)same quantity consisting of extruded soybean paste (uncased rawmedia), and four loosely crumpled balls of toilet paper. Each testspecimen shall be approximately 100 13 mm in length and 25 6 mm in diameter.” A similar media was developed by DIN (GermanIndustrial Norm/European Norm, 2006). The U.S. EnvironmentalProtection Agency's (EPA's) WaterSense program (EPA WaterSence,2014) adopted MaPs protocol and indicated that a “high efficiency”toilet should successfully and completely clear 350 g of the testspecimen from the fixture in a single flush in at least four of fiveattempts.All the above inert simulants were developed to reflect shape,size and density of real faeces. A summary of their physical properties can be found in Table 2. These simulants were mainly used forinvestigating solids movement in sewers and in drainage pipes ofbuildings and for investigating flush performance of toilet userinterfaces. Simulant faeces with varying densities and shapes asdescribed in the Bristol stool chart (Lewis and Heaton, 1997) can beproduced by modification of these physical simulants. Simulantscan be further modified to represent other type of solids found insewers such as FOGs (fats, oils and greases) by producing themfrom materials with various densities. With the rising uptake of insink food waste disposals the discharge of FOGs to sewers is widelyincreasing (Thyberg et al., 2015) and hence investigating theirtransport in sewers is of significance. These simulants do notdisintegrate and therefore are not impacted by the shear stresspresent in the system and obviously, the chemical properties arenot reflected at all. It is also important to realise that the rheologicalproperties of these simulants differ significantly from the realmaterial.Podichetty et al. (2014) evaluated the application of viscousheating for the destruction of pathogens in faeces. Heat wasgenerated within faecal simulants by applying shear stress with anextruder. They found, based on a literature review, several alternative materials displaying the same shear thinning behaviour ashuman stool, and demonstrating similar viscosity profiles withchanging shear rate. Viscosity profiles of human stools were takenfrom Woolley et al. (2013). The alternatives included contents frompig caecum (a section of the pig lower intestine) (Takahashi andSakata, 2002), content from chicken caecum (Takahashi et al.,2004), wheat flour (Podichetty et al., 2014), different types ofmashed potatoes (Podichetty et al., 2014) and simulant stool(simulant based on Susana.org.2008 (SusanA, 2008), simulant #13presented later in Table 3, viscosity profile of the simulant wasmade by Podichetty et al., 2014). While wheat flour had the closestmatch to the rheological behaviour of human faeces, they selectedred potato mash since it had a higher resemblance in terms ofmoisture content (simulant #3). Their choice of red potato mash asa faecal simulant was confirmed by its structural, thermal andviscoelastic properties (Singh et al., 2008). simulant #13 (Table 3)showed poor rheological resemblance to human stool. Rheologicalcharacteristics of the various simulants is presented in Fig. 1.Viscous heating of the red potato mash simulant was notcompared to viscous heating performance of real human stool.Further, this simulant was not tested for its density or whether itcould be representative of faeces shape. It can reasonably beassumed that this simulant will poorly represent the chemicalcharacteristics of human faeces, as it lacks important componentssuch as bacterial content, fibre, proteins and inorganic matter.Yeo and Welchel (1994) patented a synthetic faeces for simulating the dewatering rate of human stool. It was developed to beused as a substitute for real faeces in the testing and developmentof diapers. They examined 32 formulations using different components. Many of their attempts were based on a commerciallyavailable synthetic faeces, FECLONE BFPS -4 (simulant #4, Table 3)powder from Silicone Studio of Vallez Forge, Pa. FECLONE BFPS -4was reported to have a viscosity of 2276e4032 cP which is comparable to human stool, but a substantially higher dewatering rateof 524e535 gwater/m2simulant/min. In comparison, viscosity anddewatering values of human stool were reported as 3500e5500 cpand 350e400 gwater/m2faeces/min for regular stool (Tables 1 and 2).The units of the dewatering rate (g/m2/min) include m2 of material,which is determined according to the measurements procedurereported in Yeo and Welchel (1994). Since such a unit is notapplicable to be used easily for other research purposes, the authorsof this paper converted the unit to gwater/lmaterial (simulant or real stool)/min according to the

Physical properties such as shape, size, density and rheology are of importance for simulating phenomena such as faeces settling, transport in sewer pipes, dewatering, viscous heating for pathogen destruction, and physical disintegration (e.g., Butler et al., 2003; Veritec Consulting Inc. & Koeller and Company, 2010; Podichetty et al., 2014).

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