131 RBCIAMB | n.43 | mar 2017 | 131-139 José Lima de Oliveira Júnior Industrial engineering. Master in Civil and Environmental Engineering at Universidade Federal de Campina Grande (UFCG). Doctoring in Natural Resources at UFCG – Campina Grande (PB), Brazil. José Tavares de Sousa Chemical engineering. Doctor in Hydraulic and Sanitation at Universidade de São Paulo (USP). Professor of the Department of Sanitary Engineering of Universidade Estadual da Paraíba (UEPB) – Campina Grande (PB), Brazil. Andressa Muniz Santos Sanitary engineer. Master in Environmental Science and Technology at UEPB – Campina Grande (PB), Brazil. Saionara Alexandre da Silva Civil Production technician. Mastering in Natural Resources at UFCG – Campina Grande (PB), Brazil. Corresponding address: José Lima de Oliveira Júnior – Rua Plácido Aderaldo Castelo, 1.646 – Planalto – 63040‑540 – Juazeiro do Norte (CE), Brazil – E‑mail: junior@ifce.edu.br Received: 04/02/2013 Accepted: 01/27/2017 ABSTRACT Clogging tendency has been analyzed in sandy soil soakaways at pilot scale receiving either septic tank effluent (SUM1) or UASB effluent (SUM2) and also at laboratory scale (SUMB1 and SUMB2), in relation to Chemical Oxygen Demand (COD) and accumulated Suspended Solids (SS) concentrations. Clogging was also estimated, by modeling the predicted time for infiltration hydraulic failure. The laboratory results obtained with SUMB1 and SUMB2 confirmed the results obtained for SUM1 and SUM2, showing that soakaways built in sandy soils which receive effluent from septic tanks treating predominantly domestic wastewater tended to clog 58% faster than those receiving UASB effluent. The good correlation observed between the decrease in average infiltration rate and the time of operation suggests that the UASB reactor is a promising technological alternative to septic tanks as a pre‑treatment prior to effluent soil disposal for on‑site decentralized wastewater treatment systems. Keywords: decentralized treatment; UASB; septic tank; infiltration; sandy soil. RESUMO O trabalho objetivou comparar o tempo para ocorrer a colmatação de sumidouros (SUM1 e SUM2) contendo areia média, alimentados respectivamente por efluentes de tanque séptico e de um reator UASB, em escala piloto e com replicação confirmativa em escala de bancada (SUMB1 e SUMB2), quanto ao impacto da concentração acumulada de Demanda Química de Oxigênio (DQO) e dos Sólidos Suspensos Totais (SST) na colmatação do meio, em função do tempo. Estimou‑se ainda, por meio de um modelo, a predição do tempo para a falha hidráulica. A colmatação do SUMB1 à frente do SUMB2 confirmou os resultados de campo obtidos para os SUM1 e SUM2, demonstrando que sumidouros construídos e operados recebendo esgoto doméstico tratado em tanque séptico em solos arenosos tenderão a colmatar 58% mais rápido do que aqueles com tratamento prévio em reatores UASB. A boa correlação entre o decréscimo da taxa média de infiltração nos sumidouros em função de período de operação sugere o reator UASB como promissora alternativa aos tanques sépticos para disposição de efluentes no solo em sistemas descentralizados unifamiliares de tratamento de esgotos. Palavras‑chave: tratamento descentralizado; reator UASB; Tanque Séptico; infiltração; solo arenoso. DOI: 10.5327/Z2176-947820170413 INFILTRATION STUDIES ON SOAKAWAYS RECEIVING EFFLUENTS FROM SINGLE HOUSEHOLD UASB AND SEPTIC TANK REACTORS ESTUDOS DE INFILTRAÇÃO EM SUMIDOUROS RECEBENDO EFLUENTE DOMÉSTICO DE REATORES UASB E TANQUE SÉPTICO Oliveira Júnior, J.L. et al. 132 RBCIAMB | n.43 | mar 2017 | 131-139 INTRODUCTION Wastewater soil disposal is an ancient and well‑estab‑ lished worldwide practice (LOFRANO & BROWN, 2010). However, this practice, if not suitably managed, can de‑ grade natural resources by contaminating the soil ma‑ trix or even inducing its collapse (RODRIGUES; MOLINA JÚNIOR; LOLLO, 2010). Anaerobic treatment technologies of domestic waste‑ water prior to soil disposal are widely employed, with simple septic tanks being the most common solution (CRITES & TCHOBANOGLOUS, 1998; PARTEN, 2010). However, the UASB reactor and its variants have been suggested as a feasible alternative (AL‑SHAYAH & MAH‑ MOUD, 2008; SABRY, 2010; MOUSSAVI; KAZZEMBEIGI; FARZADKIA, 2010). Most household on‑site wastewater treatment sys‑ tems depend upon effluent soil infiltration for final disposal (PARTEN, 2010). However, the design pa‑ rameters for this process are still not clearly defined and this complex subject still attracts considerable discussion and demands more studies under dif‑ ferent operational conditions (SIEGRIST; MCCRAY; LOWE, 2004; BUMGARNER & MCCRAY, 2007; PEDES‑ COLL et al., 2011). Wastewater percolation, through unsaturated zone, has been reported as being con‑ trolled by a low conductivity layer in the upper lay‑ ers of the soil (RICE, 1974) and is also influenced by infiltration flow speed and substrate concentration (RICE, 1974; OKUBO & MATSUMOTO, 1983; SIEGRIST; MCCRAY; LOWE, 2004). Recent studies have confirmed the influence of the growth of the soil biomat on soil infiltration capacity when receiving effluents with accumulated total and volatile suspended solids (TSS and VSS). This devel‑ oping biomat has been considered as one of the main causes of head losses, which influence significantly the hydraulic regime under unsaturated conditions (OKUBO & MATSUMOTO, 1983; VIVIANI & LOVINO, 2004; BEAL et al., 2006; THULLNER, 2010; KIM; CHOI; PACHEPSKY, 2010). Mathematical modeling has been employed to esti‑ mate clogging in soils inundated with anaerobic reac‑ tor effluents. Predictive models have taken into consid‑ eration intervenient physical, chemical and biological parameters (BEAL et al., 2006; LEVERENZ; TCHOBANO‑ GLOUS; DARBY, 2009; THULLNER, 2010). This paper evaluates the infiltration of domestic wastewater effluents previously treated in a septic tank (ST) and a UASB reactor in relation to the impact of total suspended solids (TSS) and biochemical oxy‑ gen demand (BOD) on soil clogging using experimental soakaways packed with sand, both at laboratory and pilot scale. Predictive modeling for infiltration failure was compared to the experimental data using the pre‑ dictive model presented by Leverenz, Tchobanoglous and Darby (2009). MATERIAL AND METHODS Phase I studies During the first phase of the study, laboratory rep‑ lication scale soakaways (LREs) were fed daily for 15 days, three times a day, with a volume of 9.5 L of domestic wastewater. The LRE, as shown in Figure 1, was composed of two header tanks of 20 L for gravity feeding; two plastic cy‑ lindrical soakaways (PVC) with diameter of Φ 250 mm and useful volume of 9.5 L packed with 0.034 m3 of medium sand with effective diameter of D 10 =0.3 mm, uniformity index of C u =3.33, porosity of η%=43%, maximum diameter of D max =4.8 mm, bulk density of γ= 2.602 g.cm‑3 and saturated hydraulic conductivity of K sat =0.1925 cm.s‑1. Application of Predictive Modeling Leverenz, Tchobanoglous and Darby’s (2009) predic‑ tive model, presented in Equation 1, was applied to the prediction of failure time for sandy soil infiltra‑ tion systems: Infiltration studies on soakaways receiving effluents from single household UASB and septic tank reactors 133 RBCIAMB | n.43 | mar 2017 | 131-139 A B C From pump Maintenance door (40x40) Water level PVC pipe D = 100 mm Useful volume = 1,500 liters Soakway Infiltration zone Infiltration zone Bricks “joint free” Gravel 19 mm Medium sand 1. 67 1. 50 1. 20 0. 20 1. 10 1. 00 0. 10 0. 10 0. 50 0. 40 1. 10 Feed pipe D 25 mm 1. 82 1. 62 From pump 0. 20 Effluent pipe W.L W.L D 40 mm 0. 23 1.3 2 0. 67 0. 72 1. 10 1. 00 0. 10 Infiltration zone Infiltration zone Gravel bed D 19 mm h=0.20 Soakway Bricks “joint free” Medim sand Gravel bed 0. 40 0. 50 0. 10 1. 10 2x Feed recipientes Cap 20 l Valve D 20 mm Base Flexible pipe D 20 mm Sand column Sand column Gravel bed Gravel bed Becker 2000 ml Valved D 20 mm1 .3 0 0. 29 0. 05 0. 70 0. 13 0.25 0.25 Figure 1 – Construction features of: A) ST+SUM1; B) UASB+SUM2; C) the laboratory scale system. Oliveira Júnior, J.L. et al. 134 RBCIAMB | n.43 | mar 2017 | 131-139 1.053 6 1.318 1.120 [19.6 13.9 ( )] ⎤ ⎥ ⎦ ⎡ ⎢ ⎣[5.257 10 0.343× × × ]×i log TSSLR Tf COD Dd HLR- - = (1) Where: T f : Period of operation until failure (days); TSSLR: Total suspended solids loading rate (g TSS.m‑2. day‑1); COD i : Influent Chemical Oxygen Demand (COD) con‑ centration (mg.L‑1); Dd: Dosing frequency (doses.day‑1); HLR: Hydraulic loading rate (m.day‑1) Infiltration tests on the SUMB1 and SUMB2 soakaways The infiltration tests performed at laboratory scale under controlled conditions (SUMB1 and SUMB2) aimed to confirm the results obtained from the pilot scale soakaways operating with the Septic Tank (ST) and UASB reactor effluents to determine the varia‑ tion in infiltration flow (L.min‑1) during the test peri‑ ods of 60 minutes. The physicochemical and infiltration tests results were statistically treated by determining the dispersion and central tendency. The results were also submitted to: • descriptive statistics; • analysis of variance (ANOVA), single factor at the significance level of 5% (SOKAL & ROHLF, 1981). Phase II studies In the second phase, raw sewage was pumped via a wet well from the eastern wastewater intercep‑ tor of the city using a submersible pump (1/4 hp) to an equalization reservoir with a capacity of 1,000 L and finally pumped intermittently (using a timer control), to the pilot‑scale domestic wastewater treatment systems (DWTS), to simulate a household flow patterns with an average total flow of 270 L.d‑1. The DWTS was composed of two types of units. The first was a brick built septic tank (ST) with a working volume of 1,500 L and the second, a UASB reactor made of fiberglass with an operational vol‑ ume of 355 L. Both systems were followed by brick soakaways (Figures 1A and 1B). Infiltration tests with pilot scale soakaways SUM1 and SUM2 The pilot scale experimental soakaways (SUM1 and SUM2) were respectively fed with treated effluent from the ST and UASB reactors with an average flow of 270 L.day‑1. The infiltration tests were performed in two stages. During the first stage, 90 L of effluent were applied to the soakaways daily during a 60 minutes pe‑ riod for 10 days. In the second phase, infiltration tests were performed weekly between December 2011 and March 2012. The soakaway feed zone (SFZ) had a built length of 25 cm, a width of 100 cm and a 100 cm depth, with useful volume of 250 L. The Soil Infiltration Zone (SIZ) had a built length of 70 cm, a width of 100 cm, and a depth of 100 cm. It was packed with a basal layer of gravel/aggregate (No. 19) and an upper infiltration layer of sand 50 cm deep. During the infiltration tests, variations of recovered in‑ filtrated volume and the infiltration rates ahead of the soakaways, through a pipeline set, were monitored. The actual infiltration tests involved applying the efflu‑ ents (90 L), to each of the soakaways, and measuring the time for filling and discharging the accumulated infiltrated volume (liter by liter), over the 60 minutes test period. Figure 1 presents the construction features of both DWST [TS+SUM1 (a) and UASB+SUM2 (b)] and of the laboratory bench scale confirmatory experi‑ ments (c). Physicochemical analyses Analyses of solids and their fractions were made week‑ ly during the 15 days of laboratory scale experiments and during a period of nine months (July 2011 to April 2012) for COD and solids and their fractions for the pi‑ lot scale systems experiments using the methodology according to APHA, AWWA and WPCF (1995). Infiltration studies on soakaways receiving effluents from single household UASB and septic tank reactors 135 RBCIAMB | n.43 | mar 2017 | 131-139 RESULTS AND DISCUSSION Infiltration rates Using the results obtained at pilot scale, it was possible to create a profile of filling and discharging effluents in the SFZ, as well as the infiltration volume profile for the SIZ during the 60 minutes period. The effective volume (V ei ) of infiltrated effluent through SUM1 and SUM2 was given by Equation 2: V ei = [(L i ‑ L f ). 0.25 x 0.96] x 1000 (2) Where: V ei : Volume effectively infiltrated and discharged through the soakaways (liters); L f : Final wastewater level in the SFZ piezometer (meters); L i : Initial wastewater level in the SFZ piezometer (meters). The infiltration flow rate of the soakaways was calculat‑ ed through Equation 2, while the infiltration rate in the SFZ’s was given by Equation 3: Q i = (Δ i ‑ Δ f )/( Δt i ‑ Δ tf ) (3) Where: Q i : Infiltration effluent flow rate in the experimental soakaways (L.min‑1); Δ i : Initial recovered volume (Liters); Δ f : Final recovered volume (Liters); Δ t : Accumulated time for initial volume recovery (minutes); Δ tf : Accumulated time for final volume recovery (minutes). T i = (L i ‑ L f )/( Δt i ‑ Δ tf ) (4) Where T i : Infiltration rate in the experimental soakaways (cm.min‑1); L f : Final wastewater level in the SFZ piezometer (meters); L i : Initial wastewater level in the SFZ piezometer (meters); Δ ti : Accumulated time for initial volume recovery (minutes); Δ tf : Accumulated time for final volume recovery (minutes). Figure 2 shows the correlation between VSS accumula‑ tion and the infiltration rate in both soakaways SUM1 and SUM2, as well as between the infiltrated volumes and VSS accumulation in both soakaways SUMB1 and SUMB2. The infiltration rate variations, as well as the infiltration profile of the effluent´s edge inside SFZ of SUM1 and SUM2, are presented in Figure 3, while Figure 4 shows failure predictive modeling for SUM1, SUM2, SUMB1 and SUMB2 soakaways. SUM2 A B In fil tr ati on r at e (c m /m in ) 2.50 2.00 1.50 1.00 0.50 0.00 0 50 100 150 200 250 y= 0.0096x + 1.9703 R2=0.587 y= 0.0141x + 1.8534 R2=0.6551 Accumulated Volatile Suspended Solids (g per day) SUM1 y=10,296x-0.071 R2=0.6828 In fil tr at ed v ol um e (l) 10.2 10 9.8 9.6 9.4 9.2 9 8.8 8.6 8.4 0 2 4 6 8 10 12 14 16 Accumulated volatile suspended solids (g per day) y=9.677x-0.049 R2=0.8102 SUM2 SUM1 Figure 2 – Infiltration rate against accumulation of VSS in SUM1 and SUM2 soakaways (A) and infiltration volume against VSS accumulation in SUMB1 and SUMB2 laboratory scale soakaways (B). Oliveira Júnior, J.L. et al. 136 RBCIAMB | n.43 | mar 2017 | 131-139 Variations in COD and solids in the DWTS effluents The concentration of COD, TSS and VSS of the pre‑treated effluents prior to soil infiltration plays an important role in the magnitude of soil infiltration failure (LEVERENZ; TCHOBANOGLOUS; DARBY, 2009; PAVELICK et al., 2011). The ST and UASB reactors produced effluents with average COD concentrations of 183 mg.L‑1 and 171 mg.L‑1, respectively, which were not significantly different (p=0.333). The DWTS were fed with raw sewage (RS) with a concentration of total suspended solids (TSS) vary‑ ing between 62 and 216±45 mg.L‑1. The average con‑ centrations of TSS in the TS and UASB effluents were 32±11 mg.L‑1 and 20±7 mg.L‑1 respectively. Analysis of variance showed no significant difference between concentrations of TSS in RS and ST effluent (p=0.07258). In contrast, the reduced TSS concentration in the UASB effluent was significantly different (p=0.0463). In terms SUM2 A B In fil tr ati on r at e (l/ m in ut es ) 1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 3,25 3,00 2,75 2,50 2,25 2,00 1,75 1,50 1,25 1,00 0,75 0,50 0,25 0,00 Days SUM1 0 100 200 300 400 500 In fil tr ati on fl ow r at e (l/ m in ut es ) 0 100 200 300 400 500 600 Days Septic Tank (SUM1) UASB (SUM2) 213 days 425 days SUMB1 y=0.96 e 0.025x R2=0.5140 SUMB2 y 0.82 e 0.012x R2=0.137 270 days 574 days SUM1 y=2.511e-0.13x R2=0.658 SUM2 y=2.566 e 0.06x R2=0.615 Figure 4 –Failure modeling of experimental soakaways SUM1, SUM2 (A) and SUMB1 and SUMB2 (B). SUM2 A B In fil tr ati on fl ow r at e (li te rs /m in ut e) 3.5 3 2.5 2 1.5 1 0.5 0 2.5 2.00 1.50 1.00 0.50 0.00 Periods of observation SUM1 1 2 3 4 5 6 7 8 9 10 11 12 13 In fil tr ati on r at e (c m /m in ut e) 0 10 20 30 40 50 60 70 Test time period (minutes) SUM2 SUM1 SUM1 y=2.511 e 0.13x R2=0.658 SUM2 y=2.566 e 0.06x R2=0.615 SUM1 y=2.2847 e 0.038x R2=0.8563 SUM2 y=1.9236 e 0.035x R2=0.9464 Figure 3 – Infiltration flow rate plotted against period of operation (A) and the infiltration rate profile during the 60 minute test period (B) of the experimental soakaways SUM1 and SUM2. Infiltration studies on soakaways receiving effluents from single household UASB and septic tank reactors 137 RBCIAMB | n.43 | mar 2017 | 131-139 of VSS concentrations, the final effluents of the two DWTS, were significantly different (p=0.000118) with average concentrations for SUM1 of 29±9 mg.L‑1 and 19±6 mg.L‑1 for SUM2. Pedescoll et al. (2011) demonstrated a significant cor‑ relation between hydraulic conductivity reduction and the accumulated suspended solids loading rate. The cor‑ relation between the infiltration rate and the volatile suspended solids accumulation in the soakaways SUM1, SUM2 (Figure 2A) confirmed the influence of volatile suspended solids accumulation in hydraulic conductiv‑ ity reduction. (OKUBO & MATSUMOTO, 1983; VIVIANI & LOVINO, 2004; BEAL et al., 2006; PEDESCOLL et al., 2011). The good correlation between the decrease of infiltrated volume in time and the volatile suspend‑ ed solids accumulation in the soakaways SUMB1 and SUMB2 also confirmed the same influence (Figure 2B). Performance of experimental soakaways (SUM1 and SUM2) During test’s carrying out, the infiltration rate and the recov‑ ered volume had been monitored and presented a tendency to decrease as long as liquid percolation capacity reduced as a result of the continual decrease of hydraulic conductivity. Variations in infiltration volumes and rates The difference between the averages recovered in‑ filtration volumes of 39.62 and 62.87 L, respectively (p=3.10‑5), for SUM1 and SUM2 was significant. This sug‑ gests a greater clogging tendency for SUM1. Okubo and Matsumoto (1983) evaluated the influence of suspend‑ ed solids (SS) concentration due to synthetic effluent in‑ filtration in sand columns, showing that the increase of SS concentration reduced significantly the accumulated volume over successive days of infiltration. Figure 3A shows the correlation between the infiltra‑ tion flow rate and the operation period, with each ob‑ servation carried out for nine days in SUM1 and SUM2, along with the infiltration SIZ profile in SUM1 and SUM2 soakaways (Figure 3B). The infiltration flow rate measured in terms of percolated volume variation over time in SUM1 and SUM2 soakaways (Figure 3A) showed significant difference with average values, respectively, of 1.26 and 1.97 L.min‑1 (p=0.016). Taking into account the average loaded quantity of VSS in SUM1 and SUM2 soakaways, respectively of 19.15 and 12.38 g.day‑1 with significant difference (p‑value=0.019), it can be suggested that clogging tendency observed in TS soakaway may be related to the most substantial amount of VSS in SUM1 when compared to SUM2 accumulated VSS which has been fed with UASB effluent. The infil‑ tration rate calculated observed in SIZ piezometer, var‑ ied between 0.22 cm.min‑1 (13.45 cm.h‑1) and 2.23 cm. min‑1 (127.74 cm.h‑1) in SUM1 and from 0.20 cm.min‑1 (11.80 cm.h‑1) to 1.73 cm.min‑1 (103.78 cm.h‑1) in SUM2. Performance of SUMB1 and SUMB2 soakaways The infiltration flow rate observed in SUMB1 and SUMB2 had maximum and minimum values respectively of 0.85– 0.62 L.min‑1 and 1.01–0.59 L.min‑1, with average values of 0.717 and 0.755 L.min‑1. The infiltration failure modeling of SUMB1 and SUMB2 (R2=0.613 and 0.514 respectively), confirmed the tendency of SUMB1 to clog before SUMB2. Hydraulic failure modeling Estimates of infiltration failure were modeled for the pilot scale soakaways and confirmed by the LRE’s tests using empirically adjusted models. Equations 5 and 6 were used to model clogging for SUM1 and SUM2, while Equations 7 and 8 were used for SUMB1 and SUMB2 respectively. T i = 2.511 .e‑0.13t (5) Ti = 2.566 .e‑0.06t (6) Ti = 0.8200 .e‑0.0128t (7) Ti = 0.965 .e‑0.0248t (8) Where: Ti: Infiltration rate (L.min‑1); t: Time of operation (days); e = invariable number of the exponential function, equal to 2,71828182845904. Oliveira Júnior, J.L. et al. 138 RBCIAMB | n.43 | mar 2017 | 131-139 Based on Equations 5 and 6, infiltration failure pre‑ diction for SUM1 and SUM2 shown in Figure 4A was, respectively, of 270 and 574 days, with significant dif‑ ference between these soakaways (p=0.016). Howev‑ er, the application of Equations 7 and 8 showed a pre‑ diction a little lower than that presented formerly in Equations 5 and 6. The prediction of failure for SUMB1 and SUMB2 was, respectively of 213 and 425 days (Fig‑ ure 4B). Results suggest confirmation of gradual soil matrix obstruction in SUM1, probably due to significant effluent amount of TSS and VSS, considering that efflu‑ ent organic load is reported as responsible for a super‑ ficial bio zone formation which may reduce hydraulic conductivity by 1‑3 orders of magnitude (BUMGARNER & MCCRAY, 2007). Results obtained by applying experimental data to the Leverenz, Tchobanoglous and Darby (2009) predictive model (Equation 4) confirmed the data of both bench and pilot scale soakaway studies. The absolute differ‑ ence between all the tested experimental soakaways was around 3% and 0.91% when applying the results to the Leverenz, Tchobanoglous and Darby (2009) model. The experimental data that suggested failure due to relative clogging gave the relative difference between SUM1 and SUM2 of 53%. Whilst using the Leverenz, Tchobanoglous and Darby (2009) model, the relative difference was 58%. Likewise, at laboratory scale, the experimental results for SUMB1 and SUMB2 gave rel‑ ative differences of around 50% against 57% when ap‑ plying the Leverenz model to the soakaway data. The greater observed values for predictive hydraulic failure of the pilot scale soakaways can be explained by the greater capacity of hydraulic conductivity recovery and its corresponding infiltration rate, due to non‑con‑ trolled experimental conditions, e.g., direct solar radi‑ ation, temperature variation and wind convective ef‑ fects at the experimental site (PAVELICK et al., 2011). CONCLUSIONS The effluents produced from a household sized UASB reactor and a single household septic tank achieved similar COD total removals (p=0.1533) which may not have played an important role regarding the differenc‑ es between soakaway clogging processes. The low concentration of VSS, at a significance level of 5% (p=0.000118), in the UASB effluent compared to that of ST effluent, with respective averages of 29±9 mg.L‑1 and 19±6 mg.L‑1 could account for the low‑ er clogging tendency of the UASB soakaway during the infiltration tests. Infiltration flow rates in the pilot scale soakaways SUM1 and SUM2 showed significant differences with respective mean values of 1.26 and 1.97 L.min‑1 (p=0.016). Infiltration rates of the laboratory scale soakaways SUMB1 and SUMB2, operated under controlled condi‑ tions, has predicted the hydraulic infiltration failure of SUMB1 before SUMB2. The increased clogging tendency of SUMB1 over SUMB2 confirmed the pilot scale studies with SUM1 and SUM2, suggesting that soakaways constructed to treat the effluent from household ST’s at real scale in sandy soils will tend to clog 50 to 58% faster than those treating household UASB effluents. Prediction modeling of the clogging of soakaways in sandy soil, taking into account the influence of organ‑ ic load in terms of COD, SS and also hydraulic loading rate, and daily dosing regimens (LEVERENZ; TCHO‑ BANOGLOUS; DARBY, 2009) confirmed the clogging rates based on experimental data. The good correlation found between the average infil‑ tration rate, the number of days of operation, and the increased clogging rate (58%) obtained for the pilot scale soakaway treating ST effluent predictions, suggests that the household UASB reactor is a promising alternative to septic tanks for treatment prior to effluent soil disposal in household decentralized wastewater treatment systems. ACKNOWLEDGEMENT The authors thank CAPES and CNPq for research funding and productivity grants. Infiltration studies on soakaways receiving effluents from single household UASB and septic tank reactors 139 RBCIAMB | n.43 | mar 2017 | 131-139 REFERENCES AL‑SHAYAH, M.; MAHMOUD, N. 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