Elucidation of the Mechanism of Blockage in Sewer Pipes by Fatty Acid Deposition and Suspended Solid

water

Article Elucidation of the Mechanism of Blockage in Sewer Pipes by Fatty Acid Deposition and Suspended Solid

Toshihiko Otsuka 1, Hiroshi Yamazaki 2, Eriko Ankyu 3, Tofael Ahamed 3, Martin Anda 4 and Ryozo Noguchi 3,*

1 Saitama-Ken Environmental Analysis & Research Association, Saitama 330-0855, Japan; [email protected] 2 Faculty of Science and Engineering, Toyo University, Saitama 350-8585, Japan; [email protected] 3 Faculty of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan; [email protected] (E.A.); [email protected] (T.A.) 4 Environmental Engineering, School of Engineering and Information Technology, Murdoch University, Murdoch, WA 6150, Australia; [email protected] * Correspondence: [email protected]; Tel.: +81-29-853-4697

Received: 24 July 2020; Accepted: 13 August 2020; Published: 14 August 2020

Abstract: The objective of this study is to elucidate the mechanism by which blockages occur in sewer pipes following the deposition of fat, oil, and grease (FOG) and suspended solids (SS). In this study, a simulated wastewater flow experiment was conducted to elucidate the mechanism of sewer pipe blockage using lauric acid as fatty acid and florisil to simulate FOG and SS blockages, respectively. Unplasticized polyvinyl chloride pipes (φ = 50 mm) with a flow speed of 2 L/min and 1% inclination were used in this experiment. In “Case L & F (lauric acid florisil),” the deposition of florisil and adhesion of solids increased at the bottom of the sewer pipe over a set period. After seven days, decreases in lauric acid concentration from 1000 to 57 mg/L and in Ca2+ concentration from 18 to 0.8 mg/L were observed. FOG deposits formed solids by the saponification of lauric acid and Ca2+ from tap water. In the simulated kitchen wastewater, either lauric acid or florisil exhibited solid deposition and adhesion. Based on these findings, the blockage mechanism was elucidated to confirm FOG deposition of and SS influenced by the combination of lauric acid, Ca2+, and florisil.

Keywords: ﬂorisil; lauric acid; saponiﬁcation; wastewater; FOG deposits

1. Introduction Fat, oil, and grease (FOG) in wastewater are discharged from restaurants and food processing factories. FOG impacts the performance of wastewater treatment facilities [1], and the treated wastewater quality. Furthermore, FOG causes blockages in sewer and sewer pipes. Recently, a severe problem of blockage of the sewer pipes was observed by FOG deposits in London [2]. The direct discharge of FOG into wastewater sewage is not permitted in most municipalities. Specifically, 30 mg/L or less in Japan and 60 mg/L or less in Singapore are the regulations for oil contamination of wastewater. Therefore, it is imperative to reduce the concentration of animal fats and vegetable oils in wastewater using grease traps and/or oil collecting equipment. A grease trap requires routine maintenance to sustain the adequate performance of the oil-water separation process. However, many grease traps are poorly maintained, resulting in costly repairs and health problems after a certain period [3]. Sewer pipe blockage by FOG is significant concern around the world and requires extensive research on the FOG deposition from wastewater. The distance from the FOG sources to the deposition points in the lower sewer pipe is approximately 50 to 200 m. Besides, FOG deposits affect 25.0 to 37.5% of wastewater overflow in sewage [4]. About 60% of sewer blockages in Hong Kong are caused by FOG

Water 2020, 12, 2291; doi:10.3390/w12082291 www.mdpi.com/journal/water Water 2020, 12, 2291 2 of 12 Water 2020, 11, x FOR PEER REVIEW 2 of 13 causeddeposits by [5 FOG]. In 2004,deposits the U.S.[5]. In Environmental 2004, the U.S. ProtectionEnvironmental Agency Protection (EPA) reported Agency that (EPA) approximately reported that 3 approximatelyto 10 billion gallons 3 to 10 of untreatedbillion gallons wastewater of untreat wereed discharged,wastewater andwere about discharged, 50% of sewageand about blockages 50% of sewageare caused blockages by FOG are deposits caused inby sewer FOG deposits pipes [6]. in Environmental sewer pipes [6]. problems Environmen havetal been problems reported have due been to reportedFOG deposits due to on FOG sewer deposits pipes on that sewer are flushed pipes that away are byflushed heavy away rain andby heavy drifted rain ashore and drifted to Tokyo ashore Bay. toIn Tokyo addition, Bay. if aIn business addition, causes if a business sewer blockage causes se andwer canblockage be identified, and can itbe must identified, pay the it requiredmust pay high the requiredcost of recovery. high cost In of this recovery. regard, In sewer this regard, blockage sewer by FOG blockage deposits by FOG is a significantdeposits is concerna significant for Japanese concern forbusinesses Japanese [7 ].businesses Up to 70% [7]. of Up the sanitaryto 70% of sewage the sani overflowstary sewage that overflows occur in Malaysia that occur are in caused Malaysia by FOG. are causedIn 2010 alone,by FOG. the In wastewater 2010 alone, municipality the wastewater in Malaysia municipality received in a Malaysia total of 22,184 received blockage a total inquiries of 22,184 [8]. Theblockage concentration inquiries of[8]. FOG The inconcentration wastewater fromof FOG restaurants in wastewater in Thailand from restaurants ranged from in 730Thailand to 1100 ranged mg/L. fromBesides, 730 this to high1100 concentrationmg/L. Besides, of this FOG high is from concentration the effluents of of FOG fast-food is from restaurants the effluents [9]. Oil of and fast-food grease restaurantshave been identified [9]. Oil and as thegrease major have components been identified causing as the beach major pollution components in both causing Sydney beach and Cartagena pollution inde both Indias Sydney [10]. Thus,and Cartagena the problems de Indias and accidents [10]. Thus, involving the problems sewer blockageand accidents due to involving FOG can sewer occur blockageanywhere due in the to FOG world. can occur anywhere in the world. Moreover, veryvery fewfew research research studies studies have have been been conducted conducted to explainto explain the mechanismthe mechanism of sewer of sewer pipe pipeblockages blockages due todue the to FOG the FOG deposition. deposition. According According to the to analyticalthe analytical results results from from the FOGthe FOG samples, samples, the themain main component component was was composed composed of high of high concentrations concentrations of palmitic of palmitic acid andacidcalcium and calcium [4]. From [4]. From the FOG the FOGcharacteristics, characteristics, significant significant increases increases in the in Ca the+ content Ca+ content are observed, are observed, and the and proportion the proportion of oil of in oil FOG in FOGdeposits deposits as water as water hardness hardness increases increases [11]. When [11]. 1Wh g oilen was1 g oil added was toadded 1 L of to calcium 1 L of chloridecalcium chloride solution solutionat pH 9, FOGat pH deposits 9, FOG deposits were observed were observed on the 28th on day.the 28th Oil wasday. convertedOil was converted to free fatty to free acids fatty by alkaliacids byhydrolysis alkali hydrolysis forming FOG forming deposits FOG with deposits Ca [12 with]. The Ca metal [12]. soapThe metal formation soap indicated formation the indicated mechanism the mechanismof blockages of inside blockages the sewer inside pipe the duesewer to pipe FOG due deposition, to FOG hydrolysisdeposition, ofhydrolysis natural fat, of mineralsnatural fat, in wastewaterminerals in fromwastewater concrete from sewer concrete pipes, sewer and oxidants pipes, and in the oxidants wastewater in the [13 wastewater]. Free fatty [13]. acids Free promote fatty − 2+ 2+ acidssaponification promote (COOsaponification−) discharged (COO from) discharged the kitchen from and the calcium kitchen (Ca and) elutedcalcium from (Ca the) eluted concrete from sewer, the concreteand the FOGsewer, deposit and the due FOG to the deposit effects due of Vanto the der effe Waalscts of attraction Van der Waals and electrostatic attraction and repulsion. electrostatic FOG 2+ - 2+ repulsion.deposits are FOG reproduced deposits are by reproduced mixing the twoby mixing major the components two major (COO components− and Ca (COO) under and Ca laboratory) under laboratoryconditions conditions [12]. The proposed [12]. The mechanismproposed mechanism of FOG deposit of FOG formation deposit isformation shown in is Figure shown1 .in Figure 1.

Figure 1. ProposedProposed mechanism of fat, oil, and grease deposit formation in sewer lines [[12].12].

Wastewater discharges from restaurants and foodfood processing factories have suspended solids (SS), including including carbohydrates carbohydrates and and proteins proteins in in addition addition to oil to oiland and fat. fat.Alkaline Alkaline detergents detergents are also are used also inused dishwashers. in dishwashers. Organics Organics or chemical or chemical substances substances such such as asdetergents detergents in in wastewater wastewater are are the the main factors contributing contributing to to blockages blockages in in sewer sewer pipes. pipes. There There are are multiple multiple effects eﬀects such such as asSS SSor oil or oiland and fat infat wastewater, in wastewater, the the velocity velocity of ofwastewater, wastewater, and and th thee condition condition inside inside the the wall wall of of the the sewer sewer pipe. pipe. However, the blockage mechanism in the sewer pipe has not been elucidated accurately to enable the determination of a feasible solution. Previous Previous st studiesudies have focused focused on understanding understanding the phenomena related to blockage and dealingdealing withwith individualindividual physical-chemicalphysical-chemical and biological treatments. There is an urgent need to understand the blockage mechanism for new types of FOG and SS to find an

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Water 2020, 11, x FOR PEER REVIEW 3 of 13 is aneffective urgent needsolution to understandfor preventing the blockage blockage and mechanism wastewater fortreatment new types for food of FOG industries, and SS fast to food find an effectiverestaurants, solution and for households. preventing blockage and wastewater treatment for food industries, fast food restaurants,Therefore, and households. the purpose of this research is to determine the reason for blockage at the lab scale andTherefore, clarify the mechanism purpose of thisof blockage research formation is to determine in the sewer the reason pipe due for blockageto FOG deposition at the lab and scale SS and clarifythrough the mechanism wastewater of flows blockage from formationthe viewpoint in theof physical sewer pipe and/or due chemical to FOG conditions. deposition This and approach SS through wastewaterwill help flows to quantify from the pipe viewpoint blockage ofin physicalthe sewer and by /wastewateror chemical due conditions. to oil from This restaurants approach or will food help to quantifyprocessing pipe factories. blockage A blockage in the sewer inside by the wastewater sewer pipe duecan be to understood oil from restaurants to occur due or to food SS and processing FOG factories.deposition A blockage as demonstrated inside the sewer using pipe a simulated can be understood experiment to occurof sewer due topipe SS andblockage FOG depositionwith a as demonstratedreproducible diameter using a simulated and inclination experiment of the pipe, of sewer which pipe is commonly blockage used with in a the reproducible sewerage system diameter in Japan. and inclination of the pipe, which is commonly used in the sewerage system in Japan.

2. Materials2. Materials and and Methods Methods

2.1. Equipment2.1. Equipment for thefor the Simulated Simulated Experiment Experiment of of Sewer Sewer PipePipe BlockageBlockage FOG deposits, SS, wastewater temperature, wastewater speed, and microorganisms interact to FOG deposits, SS, wastewater temperature, wastewater speed, and microorganisms interact to cause blockage of sewer systems in kitchens, restaurants, and food factories. The simulated cause blockage of sewer systems in kitchens, restaurants, and food factories. The simulated experiment experiment for sewer pipe blockage was conducted using the artificial equipment involving sewer for sewerpipes pipeto find blockage the direct was relationship conducted between using the the interaction artificial equipment of FOG deposit involving and SS sewerand the pipes blockage to find the directin sewer relationship pipes. between the interaction of FOG deposit and SS and the blockage in sewer pipes. TransparentTransparent unplasticized unplasticized polyvinyl polyvinyl chloride chloride pipespipes (uPVC pipe, pipe, ϕφ = =5050 mm) mm) and and 1%1% pipe pipe inclinationinclination were were used used for for the the sewer sewer pipes pipes of of the the equipment. equipment. A A screening screening mesh mesh made made of ofplastic plastic with with openingsopenings of 5 of mm 5 mm was was set set between between the the final final parts parts ofof thethe sewer pipe pipe to to create create blockages blockages in inthe the sewer sewer pipepipe easily. easily. The The equipment equipment was was set set up up inin an experi experimentalmental room room with with a room a room temperature temperature of 20 of°C. 20 A ◦C. A pumppump was was installed installed to to circulate circulate the the simulatedsimulated wastewaterwastewater in in the the equipment. equipment. The The pump pump was was operated operated intermittently,intermittently, with with a pumping-on a pumping-on time time of of 30 30 min min and and aa pumping-opumping-offff timetime of of 30 30 min min (total (total circulated circulated volumevolume 60 L 60/time). L/time). The The experiment experiment was was performed performed atat a velocity of of 8 8L/min L/min for for simulated simulated wastewater. wastewater. As a result, the velocity of the wastewater was too high to check the blockage in the sewer pipe. There As a result, the velocity of the wastewater was too high to check the blockage in the sewer pipe. was a possibility of blockage in the sewer pipe where the wastewater flow was weak. Previous studies There was a possibility of blockage in the sewer pipe where the wastewater flow was weak. Previous have reported that the blockage in sewer pipes occurred at a point away from the FOG source [4]. studiesHence, have the reported adopted that velocity the blockage of the insimulated sewer pipes wast occurredewater was at a2 pointL/min. away Four from cameras the FOGwere source set to [4]. Hence,observe the adoptedthe flow of velocity change offorthe simulated simulated wastewater wastewater in the wassewer 2 pipe L/min. (Figure Four 2). cameras were set to observe the flow of change for simulated wastewater in the sewer pipe (Figure2).

Figure 2. Cont.

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Figure 2. Outline of the equipment for simulated experiment to elucidate blockage mechanism in the Figure 2. Outline of the equipment for simulated experiment to elucidate blockage mechanism in the sewer pipe: (A) top view and (B) side view. sewer pipe: (A) top view and (B) side view. 2.2. Combination of Simulated Wastewater 2.2. Combination of Simulated Wastewater Tap water (SS concentration 0 mg/L, Ca2+ concentration 18 mg/L, oil concentration 0 mg/L), lauric Tap water (SS concentration 0 mg/L, Ca2+ concentration 18 mg/L, oil concentration 0 mg/L), lauric acid (C H O ) as saturated and unsaturated fatty acids for FOG deposit and florisil as SS of vegetable acid 12(C1224H24O2 2) as saturated and unsaturated fatty acids for FOG deposit and florisil as SS of vegetable scrapscrap were were used used to to prepare prepare 2020 LL ofof simulatedsimulated wast wastewaterewater for for the the experiment. experiment. Lauric Lauric acid acidis also is also includedincluded in in commercial commercial detergents detergents forfor dishwashers or or soaps soaps [14]. [14 ]. ThreeThree types types of of simulated simulated wastewaterwastewater were were used: used: Case Case L & L &F (lauric F (lauric acid acid and andflorisil), florisil), Case CaseF F (florisil),(florisil), and and Case Case L L (lauric (lauric acid).acid). The concentrations concentrations of oflauric lauric acid acid and and florisil florisil were were simulated simulated in in generalgeneral kitchen kitchen wastewater wastewater usingusing a disposer perfor performancemance test test in inJapan Japan [15]. [15 These]. These concentrations concentrations werewere adjusted adjusted to to 1000 1000 mgmg/L/L lauric acid acid and and 1750 1750 mg/L mg florisil/L florisil concentration. concentration. Ca2+ was Ca 2included+ was included in the in tap water as a mineral. The temperature in simulated wastewater was maintained at 40 to 45 °C the tap water as a mineral. The temperature in simulated wastewater was maintained at 40 to 45 ◦C duringduring the the experiment. experiment. TheThe reasons were were that that the the melting melting point point of oflauric lauric acid acid was wasapproximately approximately 45 °C, the simulated wastewater was supposed to be similar to kitchen wastewater from the 45 C, the simulated wastewater was supposed to be similar to kitchen wastewater from the restaurant, ◦restaurant, and according to our experience, kitchen wastewater has a temperature of approximately and according to our experience, kitchen wastewater has a temperature of approximately 40 C. 40 °C. The temperature was maintained during the experiment using a small heater. Simulated ◦ Thewastewater temperature is shown was maintained in Table 1. duringThe physical the experiment properties usingof lauric a small acid heater.[16] and Simulated florisil [17] wastewater used in is shownthis experiment in Table1. Theare shown physical in Table properties 2. of lauric acid [ 16] and florisil [17] used in this experiment are shown in Table2. Table 1. Concentration of the types of simulated wastewater used in the experiment. Table 1. Concentration of the types of simulated wastewater used in the experiment. Lauric Acid Florisil Ca2+ Sample ConcentrationLauric Acid Concentration Florisil ConcentrationCa2+ Sample 1000Concentration [mg/L] 1750Concentration [mg/L] 18Concentration [mg/L] Case L & F 1000〇 [mg/L] 1750〇 [mg/L] 18〇 [mg/L] 〇〇 CaseCase LF & F - 〇〇 CaseCase L F ### - - Case L ##- # #

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Table 2. Physical properties of the lauric acid and ﬂorisil for the experiment.

Physical Properties Lauric Acid Florisil

Boiling point [◦C] 298 - Melting point [◦C] 45 Over 900 Solubility in water at 25 ◦C [mg/L] 4.81 Insoluble Diameter [µm] - 150 to 250 µm

2.3. Experimental Procedure to Simulate Sewer Pipe Blockage The experimental conditions of the velocity or temperature of the simulated wastewater and other parameters are listed in Table3. There are cases where wastewater ﬂows continuously in actual sites. Although the experiment was performed in batch type, the operation of the equipment was brought closer to the real site under the experimental conditions shown in Table3.

Table 3. Experimental parameters and conditions considering diﬀerent parameters of wastewater ﬂow conditions.

Parameters Condition Reason The blockage in the sewer pipe occurred at the point located away from the wastewater Velocity of wastewater 2 source. There is a possibility of a blockage (L/min) occurring in the sewer pipe where wastewater ﬂow is in weak condition. The simulated wastewater was supposed to represent kitchen wastewater from the Wastewater temperature ( C) 40–45 ◦ restaurant, and its temperature in the actual site was around 40 ◦C. Unplasticized polyvinyl A material of sewer pipe using at the actual Pipe material chloride site was used. Pipe diameter [µm] 50 Sewer pipe diameter using in general Pipe inclination [%] 1 Sewer pipe inclination using in general Pumping operation intermittent operation Findings in the practical ﬁeld

Seven days was set as the operating period. Images were taken at each 10 min interval to observe changes, deposition, and adhesion inside the sewer pipe. Low concentrations of lauric acid and florisil were reported for the wastewater quality on the 2nd day. During this period, the need to add lauric acid and florisil was assessed to cause a blockage. Then, lauric acid and florisil of additional half of the amount (lauric acid 10 g and florisil 17.5 g) were added into the wastewater on the 2nd day of the experiment. Besides, tap water was added until the amount of simulated wastewater reached 20 L to compensate for the evaporated wastewater. The concentrations of lauric acid, florisil, and Ca2+ were analyzed on the 1st, 2nd, and 7th day. FOG deposits were formed on the sewer pipe on the 5th day, and therefore the wastewater quality was not analyzed to consider how sampling affected the blockage. This experiment included water quality analysis and was performed twice to confirm the reproducibility of the findings. The lauric acid concentration was analyzed by adding 60 mL of hexane to 1 L of simulated wastewater, stirring the simulated wastewater at 250 rpm for 10 min, extracting the hexane layer, and volatilizing hexane completely at 80 ◦C. The concentration was calculated by dividing the lauric acid weight by the amount of the sample. In the analysis of the concentration of florisil, 200 mL of simulated wastewater using glass fiber filter paper (pore size one µm), whose weight was measured, was filtered, and the filter paper was dried at 110 ◦C for 2 h and weighed again. The concentration was calculated by dividing the measured weight by the amount of sample water. The concentration of Ca2+ in tap water was measured by adding 2 mL of hydrochloric acid (1 + 1) to the simulated wastewater filtered using a membrane filter (pore size 0.45 µm), and then topping up the sample to 100 mL and adding 10 mL of lanthanum (La, 50 g/L). This solution was then analyzed by atomic absorption spectrometry. Water 2020, 11, x FOR PEER REVIEW 6 of 13 Water 2020, 11, x FOR PEER REVIEW 6 of 13 of Ca2+ in tap water was measured by adding 2 mL of hydrochloric acid (1 + 1) to the simulated 2+ ofwastewater Ca in tap filtered water using was measureda membrane by filteradding (pore 2 mL size of 0.45 hydrochloric μm), and then acid topping (1 + 1) upto thethe simulatedsample to wastewater100 mL and filtered adding using 10 mL a membraneof lanthanum filter (La, (pore 50 g/L).size 0.45 This μ solutionm), and thenwas toppingthen analyzed up the bysample atomic to Water100absorption mL2020 and, 12 ,spectrometry. 2291adding 10 mL of lanthanum (La, 50 g/L). This solution was then analyzed by atomic6 of 12 absorption spectrometry. 3. Results 3. Results 3.1. Results for Case L & F (Lauric Acid and Florisil) 3.1. Results for Case L & F (Lauric Acid and Florisil) The analysis of Case L & F (lauric acid and florisil) was conducted from 8 May 2018 to 15 May The analysis of Case L & F (lauric acid and florisil) was conducted from 8 May 2018 to 15 May 2018.The From analysis the results, of Case solid L &deposits F (lauric in acidthe sewerand fl pipeorisil) were was observedconducted at fromthe observation 8 May 2018 point to 15 2 May and 2018. From the results, solid deposits in the sewer pipe were observed at the observation point 2 and 4 2018.4 of the From straight the results, pipe. The solid deposition deposits inprocess the sewer of solids pipe wereat the observed observation at the point observation 4 is shown point in Figure 2 and of the straight pipe. The deposition process of solids at the observation point 4 is shown in Figure3, 43, ofand the the straight clogging pipe. condition The deposition of the sewer process pipe of after solids the at 7th the day observation is shown pointin Figure 4 is 4.shown in Figure and3, and the the clogging clogging condition condition of of the the sewer sewer pipe pipe after after the the 7th 7th day day is shownis shown in Figurein Figure4. 4.

Figure 3. Deposition process of solids in the sewer pipepipe inin thethe simulatedsimulated experimentexperiment (Observation(Observation Figure 3. Deposition process of solids in the sewer pipe in the simulated experiment (Observation point 4). point 4).

Figure 4. Clogging inside the sewer pipe on the 7th dayday inin thethe simulatedsimulated experimentexperiment (Observation(Observation pointFigure 4). 4. Clogging inside the sewer pipe on the 7th day in the simulated experiment (Observation point 4). The deposition of florisilflorisil was observedobserved on thethe pipepipe bottombottom afterafter 11 hh ofof thethe experiment.experiment. The depositionThe deposition process of of florisilflorisil florisil andand was solidssolids observed waswas foundfound on th atate thethepipe bottombottom bottom ofof theafterthe pipepipe 1 h 2424 of h.h. the The experiment. deposition The and adhesiondeposition of process solid were of florisil observed and aftersolids 48 was h. Afterfound 72 at h, the an bottom increase of inthe the pipe thickness 24 h. The of deposition adherence wasand observedadhesion of over solid time. were There observed was no after overflow overflow 48 h. After of the 72 simulated h, an increase wastewater in the thickness from the of sewer adherence pipe, since was theobserved operation over of time. the pumpThere waswas atno a overflow constant dischargedischaof the rgesimulated flowflow rate wastewater of 2 LL/min./min. from The the speed sewer of wastewater pipe, since inthe the operation unblocked of the part pump increased, was at and a constant therefore, discha therge simulated flow rate wastewater of 2 L/min. flow The was speed not of a ffwastewaterected. The results of water quality analysis at the 1st and 7th day in the experiment are shown in Figure5. Water 2020, 11, x FOR PEER REVIEW 7 of 13

in the unblocked part increased, and therefore, the simulated wastewater flow was not affected. The results of water quality analysis at the 1st and 7th day in the experiment are shown in Figure 5. Water quality analysis on the 2nd day showed a decrease in the concentration of lauric acid, florisil, and Ca2+. The solids adhering to the pipe were estimated by the saponification of lauric acid and Ca2+. Moreover, the florisil concentration decreased to 100 from 1750 mg/L in the sewer pipe. Afterward, the amount of solids and florisil increased with the addition of lauric acid and florisil and clogged the pipe. Water quality analysis on the 7th day showed a decrease in the concentrations of lauric acid, florisil, and Ca2+ and the deposition of florisil, and the saponification of lauric acid and Ca2+ persisted. Case L & F was conducted twice, and the results of water quality and blockage in the sewer pipe were the same in both experiments. On an average, water quality results for the 2nd and Water 7th2020 day, 12, are 2291 shown in Table 4. 7 of 12

Figure 5. Comparison of water quality analytical findings for Case L & F (lauric acid and florisil) for differentFigure days. 5. Comparison of water quality analytical findings for Case L & F (lauric acid and florisil) for different days. Water quality analysis on the 2nd day showed a decrease in the concentration of lauric acid, Table 4. Average of water quality results. florisil, and Ca2+. The solids adhering to the pipe were estimated by the saponification of lauric acid 2+ Average(mg/L) and Ca . Moreover, the florisil concentrationItems decreased to 100 from 1750 mg/L in the sewer pipe. Afterward, the amount of solids and florisil increased2nd Day with the 7th addition Day of lauric acid and florisil and clogged the pipe. Water qualityLauric analysis acid on the 7th 123 day showed 60 a decrease in the concentrations of lauric acid, florisil, and Ca2+ andFlorisil the deposition of104 florisil, and the83 saponification of lauric acid and Ca2+ 0.47 0.64 Ca2+ persisted. Case L & F was conducted twice, and the results of water quality and blockage in the sewer3.2. pipe Result were for Case the sameF (florisil) in both experiments. On an average, water quality results for the 2nd and 7th day are shown in Table4. The experiment with Case F was conducted from 22 May 2018 to 29 May 2018. The image of the sewer pipe on the 7th day is shown in Figure 6. The deposition of florisil on the pipe was observed; Table 4. Average of water quality results. however, the formation of the deposits was not observed. Moreover, neither the adhesion nor deposition of solids in the pipe was observed. The findingsAverage(mg of the water/L) quality analysis are shown in Figure 7. Water qualityItems analysis on the 2nd and 7th days in the experiment showed that the florisil 2nd Day 7th Day concentration decreased to 89 from 1750 mg/L on the 7th day. This result confirmed the deposition of florisil on the pipe asLauric observed acid in this experiment. 123 The Ca2+ concentration 60 increased from 18 to 27 mg/L. The amount of simulatedFlorisil wastewater decreased, 104 and the Ca2+ concentration 83 was increased by Ca2+ 0.47 0.64

3.2. Result for Case F (florisil) The experiment with Case F was conducted from 22 May 2018 to 29 May 2018. The image of the sewer pipe on the 7th day is shown in Figure6. The deposition of florisil on the pipe was observed; however, the formation of the deposits was not observed. Moreover, neither the adhesion nor deposition of solids in the pipe was observed. The findings of the water quality analysis are shown in Figure7. Water quality analysis on the 2nd and 7th days in the experiment showed that the florisil concentration decreased to 89 from 1750 mg/L on the 7th day. This result confirmed the deposition of florisil on the pipe as observed in this experiment. The Ca2+ concentration increased from 18 to 27 mg/L. The amount of simulated wastewater decreased, and the Ca2+ concentration was increased by evaporation from the tank because the sample temperature was maintained between 40 and 45 ◦C. Evaporation of simulated wastewater was observed for all experiments. Water 2020, 11, x FOR PEER REVIEW 8 of 13 Water 2020, 11, x FOR PEER REVIEW 8 of 13

Waterevaporationevaporation2020, 12, 2291from from the the tank tank becausebecause thethe samplesample temperature temperature was was maintained maintained between between 40 and 40 45and °C. 45 8 of°C. 12 EvaporationEvaporation of of simulated simulated wastewaterwastewater waswas observed observed for for all all experiments. experiments.

Figure 6. Sewer pipe blockage condition on the 7th day from the simulated experiment (Observation Figurepoint 6.4). SewerSewer pipe pipe blockage condition on the 7th day from the simulated experiment (Observation point 4).

Figure 7. Comparison of water quality analytical results for Case F for diﬀerent durations. Figure 7. Comparison of water quality analytical results for Case F for different durations. 3.3. Result for Case L (Lauric Acid)

3.3.The Result experiment for Case L on(Lauric lauric Acid) acid was conducted from 7 June 2018 to 14 June 2018. The images of the sewer pipeTheFigure experiment on the 7. Comparison 2nd on day lauric and of acid 7thwater was days quality conducted are analytical shown from in re Figure7sults June for 82018. Case Neither to F14 for June deposition different 2018. durations.The nor images adhesion of of solidsthe sewer on the pipe pipe on was the 2nd observed day and on 7th 2nd days day. are However, shown in theFigure deposit 8. Neither of solids deposition and a littlenor adhesion adherence to 3.3. Result for Case L (Lauric Acid) theof pipe solids were on the observed pipe was on observed the 7th day.on 2nd The day. water However, quality the analytical deposit of results solids and are showna little adherence in Figure 9. to ThetheWater pipe experiment quality were observed was on not lauric on analyzed the acid 7th was day. on theconducted The 2nd wate dayr quality from because 7 analytical June deposits 2018 results to and 14 adhesionare June shown 2018. ofin The solidFigure images were 9. not of theobserved. sewer pipe Moreover, on the the2nd water day and quality 7th days analysis are shown required in aFigure sample 8. volumeNeither correspondingdeposition nor toadhesion 10% of ofthe solids total on sample. the pipe In thiswas regard,observed from on 2nd the resultsday. Ho ofwever, the water the deposit quality of analysis solids and on thea little 7th adherence day of the toexperiment, the pipe were the concentrationobserved on the of lauric7th day. acid The concentration water quality increased analytical from results 1000 are to 1158shown mg in/L, Figure and the 9. concentration of Ca2+ decreased from 18 to 0.4 mg/L. Decreasing the Ca2+ concentration was due to the reaction of lauric acid and Ca2+. For this reason, the solids were considered to deposit FOG from lauric acid and Ca2+. Moreover, an increase in lauric acid concentration was supposed to be due to the evaporation of water and the addition of lauric acid; however, it was lower than the concentration when additional amounts were added.

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Figure 8. Sewer pipe after 2nd and 7th days (observation point 4, left: 2nd day, right: 7th day). Figure 8. Sewer pipe after 2nd and 7th days (observation point 4, left: 2nd day, right: 7th day).

Figure 9. Comparison of water quality analysis in Case L during different periods. Figure 9. Comparison of water quality analysis in Case L during different periods. 4. Discussion Water quality was not analyzed on the 2nd day because deposits and adhesion of solid were not 4.1. Discussion on the Experimental Results observed. Moreover, the water quality analysis required a sample volume corresponding to 10% of Figure 9. Comparison of water quality analysis in Case L during different periods. theFormation total sample. of FOG In this deposits regard, byfrom He the et al.results (2013) of the [12 ]water was confirmedquality analysis after on 2 days the 7th in theday laboratoryof the experiment, the concentration of lauric acid concentration increased from 1000 to 1158 mg/L, and the usingWater the grease quality interceptor was not analyzed wastewater on mixedthe 2nd with day Ca because concentration deposits of and 50 adhesion mg/L. The of FOG solid deposition were not concentration of Ca2+ decreased from 18 to 0.4 mg/L. Decreasing the Ca2+ concentration was due to observed.and FOG adhesionMoreover, on the the water sewer quality pipe was analysis observed required 48 h aftera sample the start volume of the corresponding experiment. Therefore,to 10% of the reaction of lauric acid and Ca2+. For this reason, the solids were considered to deposit FOG from the speed of formation of FOG deposits in this experiment was comparable with the result reported by the laurictotal sample.acid and InCa this2+. Moreover, regard, from an increase the results in lauric of the acid water concentration quality analysiswas supposed on the to 7th be dueday toof the He et al. (2011) [18]. experiment,the evaporation the concentration of water and theof lauric addition acid of concentrationlauric acid; however, increased it was from lower 1000 than to the 1158 concentration mg/L, and the concentrationwhenClogging additional of in theCa amounts2+ sewer decreased pipe werewas fromadded. observed 18 to 0.4 in mg/L. Case LDecreasing & F and was the not Ca observed2+ concentration in Case was L and due Case to theF. FOG reaction deposition of lauric and acid FOG and adhesion Ca2+. For in this the reason, pipe were the solids observed were at considered the points 2to and deposit 4. On FOG the otherfrom laurichand,4. Discussion atacid the and observation Ca 2+. Moreover, point 3, an FOG increase deposits in lauric were observedacid concentration to adhere towas the supposed sidewall to of thebe due sewer to pipethe evaporation but did not of form water any and deposits. the addition The of deposition lauric acid; of however, florisil at it the was bottom lower ofthan the the sewer concentration pipe was whenobserved4.1. additional Discussion before FOGonamounts the depositionExperimental were added. at Results the observation points 2 and 4. However, this was not observed at the pointFormation 3. Thus, of FOG the FOG deposits deposits by He flowing et al. (2013) were [12] not was caught confirmed by florisil after 2 and days did in the not laboratory deposit at the 4.bottom Discussionusing of the the grease sewer pipeinterceptor of observation wastewater point mixed 3. The with blockage Ca concentration was not observed of 50 mg/L. when FOGThe FOG deposits were not detected by florisil like Case L and observation point 3. From the above, solutions of blockage

4.1.include Discussion removing on the SS Experimental before wastewater Results started ﬂowing into the sewer pipe and/or introducing a new sewer pipe which can cancel SS deposit on the bottom. Formation of FOG deposits by He et al. (2013) [12] was confirmed after 2 days in the laboratory using the grease interceptor wastewater mixed with Ca concentration of 50 mg/L. The FOG

Water 2020, 12, 2291 10 of 12

Moreover, Case L & F was conducted twice, and the results of water quality and blockage in the sewer pipe were the same in both experiments. Therefore, this experiment was stable and showed reproducibility of the blockage in the sewer pipe. Considering that the amount of water required for wastewater quality analysis affects the experimental results, the wastewater quality analysis was not performed each day during the experiment. However, an increase in the amount of FOG deposition was observed with progress of the time in Case L & F. Neither FOG deposition nor FOG adhesion on the pipe was observed on the 2nd day in Case L. However, a small amount of FOG deposition and FOG adhesion to the pipe was observed on the 7th day. From the above, it was estimated that the saponification of lauric acid and Ca2+ proceeded over time, and the concentrations of lauric acid and Ca2+ gradually decreased. On the other hand, in Case F, since the deposition of florisil was observed immediately after the start of the experiment, it was estimated that the florisil concentration decreased shortly after the addition of florisil, and then became constant. The evaporation of simulated wastewater was maintained between 40 and 45 ◦C, which led to an increase in the concentration of lauric acid, florisil, and Ca2+. In the actual site, the temperature of kitchen wastewater flows at an approximate temperature of 40 ◦C or more and fatty acid, SS, and Ca2+ are concentrated by the evaporation of wastewater in the sewer pipe. Therefore, wastewater temperature is a critical factor that influences the blockage in the sewer pipe. Moreover, fatty acids become more viscous as the temperature decreases [19]. Therefore, in the vicinity of the wastewater source, the temperature of the wastewater did not decrease and the fatty acid had a low viscosity. However, away from the wastewater source, the temperature of the wastewater was dropped, and the viscosity of fatty acids increased, which made it easier for the fatty acids to adsorb on the SS and/or the side surfaces of sewer pipes, resulting in easier clogging.

4.2. Sewer Pipe Blockage Mechanism The blockage mechanism inside the sewer pipe by SS and FOG deposits from the saponification of fatty acids and Ca2+ can be explained as follows: first, the water flow is weakened away from the sewerage source at the corners of the sewer pipe; second, SS is deposited because of the weakening of the flow; third, FOG depositions occur due to the saponification of fatty acid and Ca2+ in wastewater; fourth, FOG deposition is accelerated due to catching on SS and adhering to the sewer pipe; and fifth, wastewater flow becomes weak, and deposition of SS and FOG adhesion increases. Finally, sewer pipe blockageWater 2020 occurs., 11, x FOR The PEER blockage REVIEW mechanism in the sewer pipe is observed from experimental11 of 13 results as shown in Figure 10.

Top view of sewer pipe Fatty acid Ca2+ SS

Ⅰ Wastewater flow is weakened Ⅱ Deposition of SS around the Ⅲ Formation of FOG deposits by away from the sewerage source at corner or away from the sewerage saponification of fatty acid and Ca2+ the corner source

FOG deposits

Ⅵ Ⅴ Sewer pipe is blocked Wastewater flow becomes weak and SS Ⅳ FOG deposits is caught on SS deposits and FOG deposits adheres more Figure 10. Elucidated mechanism of the sewer pipe blockage. Figure 10. Elucidated mechanism of the sewer pipe blockage. From the experimental results, it was concluded that the blockage could be prevented by combiningFrom the the experimental three elements: results, lauric it acid, was Ca concluded2+, and florisil. that It the has blockage been shown could that bethe prevented FOG deposits by combining the threewith the elements: highest SS lauricconcentration acid, Ca had2+ the, and highest ﬂorisil. oil content, It has and been old shown sewers thatwith thegentle FOG slopes deposits have with the highesta possible SS concentration accumulation had of theSS and/or highest FOG oil content,and contribute and old to sewerstheir high with concentrations gentle slopes in havethe a possible wastewater [11]. Therefore, the blockage mechanism observed in this experiment result can simulate blockage incidents and already reported by FOG deposits at the actual sites.

4.3. Future Subject The approximate cost of restoration from the blockage of the sewer pipe is 1000 to 10,000 USD in Japan. Therefore, the economic losses for businesses and administrations is substantial. In this regard, understanding of the sewer pipe blockage mechanism is very important to improve these situations and decrease the additional cost for wastewater management. In the actual sites, various factors such as the use of dish-washing detergents, changing of wastewater temperature, different particle size SS, and various types of fatty acids are acting in combination with each other. This experimental condition is the ideal condition consisting of a kind of fatty acid and SS, and it is only one of these factors thought to affect sewer pipe blockage. However, it is expected that the findings of this study will lead to the elucidation of the sewer pipe blockage mechanism occurring at the actual site.

5. Conclusions The mechanism of blockage of the sewer pipe was investigated using an experiment simulating wastewater flow, assuming the properties of kitchen wastewater are maintained. From the findings of this research, the following conclusions are drawn:

(1) In Case L & F, a decrease in the concentration of lauric acid and Ca2+ was observed, and the formation of FOG deposits due to saponification of lauric acid and Ca2+ was observed. (2) In Case F, the concentration of florisil was observed to have decreased due to the accumulation on the bottom of the pipe, but the drainpipe was not blocked. (3) In Case L, a decrease in the concentration of lauric acid and Ca2+ was observed, and the formation of FOG deposits due to saponification of lauric acid and Ca2+ was observed. Although some FOG deposition and FOG adhesion to the pipe was observed, it did not lead to blockage. (4) In this experiment, sewer pipe blockage was not observed in the simulated wastewater lacking either SS or lauric acid. Therefore, it was shown that pipe blockage due to FOG deposits requires the co-existence of fatty acids and SS. (5) The sewer pipe blocking mechanism using SS and FOG deposits was proven from the reports of the actual sites.

Water 2020, 12, 2291 11 of 12 accumulation of SS and/or FOG and contribute to their high concentrations in the wastewater [11]. Therefore, the blockage mechanism observed in this experiment result can simulate blockage incidents and already reported by FOG deposits at the actual sites.

4.3. Future Subject The approximate cost of restoration from the blockage of the sewer pipe is 1000 to 10,000 USD in Japan. Therefore, the economic losses for businesses and administrations is substantial. In this regard, understanding of the sewer pipe blockage mechanism is very important to improve these situations and decrease the additional cost for wastewater management. In the actual sites, various factors such as the use of dish-washing detergents, changing of wastewater temperature, different particle size SS, and various types of fatty acids are acting in combination with each other. This experimental condition is the ideal condition consisting of a kind of fatty acid and SS, and it is only one of these factors thought to affect sewer pipe blockage. However, it is expected that the findings of this study will lead to the elucidation of the sewer pipe blockage mechanism occurring at the actual site.

5. Conclusions The mechanism of blockage of the sewer pipe was investigated using an experiment simulating wastewater ﬂow, assuming the properties of kitchen wastewater are maintained. From the ﬁndings of this research, the following conclusions are drawn:

(1) In Case L & F, a decrease in the concentration of lauric acid and Ca2+ was observed, and the formation of FOG deposits due to saponification of lauric acid and Ca2+ was observed. (2) In Case F, the concentration of florisil was observed to have decreased due to the accumulation on the bottom of the pipe, but the drainpipe was not blocked. (3) In Case L, a decrease in the concentration of lauric acid and Ca2+ was observed, and the formation of FOG deposits due to saponification of lauric acid and Ca2+ was observed. Although some FOG deposition and FOG adhesion to the pipe was observed, it did not lead to blockage. (4) In this experiment, sewer pipe blockage was not observed in the simulated wastewater lacking either SS or lauric acid. Therefore, it was shown that pipe blockage due to FOG deposits requires the co-existence of fatty acids and SS. (5) The sewer pipe blocking mechanism using SS and FOG deposits was proven from the reports of the actual sites.

Author Contributions: Conceptualization, T.O. and R.N.; methodology, T.O., H.Y., E.A. and R.N.; validation, T.O. and H.Y.; formal analysis, T.O., H.Y. and R.N.; writing—original draft preparation, T.O.; writing—review and editing, T.A., M.A. and R.N.; visualization, T.O. and R.N.; supervision, R.N.; project administration, R.N. All authors have read and agreed to the published version of the manuscript. Funding: This research and the APC were funded by Japan Society for the Promotion of Science (JSPS) KAKENHI, Grant Number 20H03104. Acknowledgments: This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI, Grant Number 20H03104. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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