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