key: cord-0890728-m41z3rwe
authors: He, Jiajie; Dougherty, Mark; Chen, Zhongbing
title: Numerical assessment of a soil moisture controlled wastewater SDI disposal system in Alabama Black Belt Prairie
date: 2020-09-02
journal: Chemosphere
DOI: 10.1016/j.chemosphere.2020.128210
sha: 4bde3dfe2dcff44ab9f7919f2e41702bfc654145
doc_id: 890728
cord_uid: m41z3rwe

To promote the environmental sustainability of rural sanitation, a soil moisture controlled wastewater subsurface drip irrigation (SDI) dispersal system was field tested in the Black Belt Prairie of Alabama, USA. The soil moisture control strategy was designed to regulate wastewater disposal timing according to drain field conditions to prevent hydraulic overloading and corresponding environmental hazard. CW2D/HYDRUS simulation modeling was utilized to explore difficult-to-measure aspects of system performance. While the control system successfully adapted hydraulic loading rate to changing drain field conditions, saturated field conditions during the dormant season presented practical application challenges. The paired field experiment and simulation model demonstrate that soil biofilm growth was stimulated in the vicinity of drip emitters. Although biofilm growth is critical in maintaining adequate COD and [Formula: see text] removal efficiencies, the efficient removal of biodegradable COD itself by soil biofilm limits denitrification of formed [Formula: see text] . Furthermore, stimulated soil biofilm growth can create soil clogging around drip emitters, which was discerned in the field experiment along with salt accumulation, both of which were verified by simulation. Comparable modeling of system performance in sand and clay media demonstrate that the placement of soil moisture sensors within the drain field can have pronounced impacts on system hydraulic performance, depending on the soil permeability. Overall, the soil moisture control strategy tested is shown as a viable supplemental technology to promote the environmental sustainability of rural sanitation systems.

It is common sense that domestic wastewater should undergo proper treatment 25 before discharge into the natural environment (Henze et al., 2008) . The recent outbreak of 26 viral pneumonia (COVID-19) further heightens the importance of safe disposal of 27 household wastewater (Yeo et al., 2020) . However, rural areas across the globe often lack 28 necessary access to safe sanitation (Islam and Smith, 2019) . Although the United Nations 29 recognizes access to sanitation as a human right equal to safe drinking water (G.A. 2010, 30 2015), disparities, inequalities, and neglect are increasingly used to describe differences 31 in access between rural and urban populations (Kaminsky and Javernick-Will, 2014) . 32

Society is built upon the efficient cooperation between urban and rural sectors, and thus 33 rural sanitation deserves more targeted study with respect to its economic and 34 environmental sustainability (Ji et al., 2018; Kaminsky and Javernick-Will, 2014) . 35

Although the Human Rights Act does not identify a specific technology for sanitation in 36 rural areas, natural-based onsite wastewater treatment systems (OWTS) have been widely 37 used in rural areas of both developing and developed countries (Jantrania and Gross, 38 2006) . The successful application of OWTS demands favorable site conditions such as 39 good drainage and zero or rare flooding occurrence (NRCS, 1993) . 40

The Black Belt Prairie in the Southern US, which runs through Mississippi and 41

Alabama is approximately 310 miles long and up to 25 miles wide. This unique region 42

showcases the geological and climate challenges inherent in many rural areas worldwide 43 with similar limitations. Despite its historical glory before the US Civil War, the current 44 economic and social development of the Black Belt Prairie falls behind the national 45 standard (Wedgworth and Brown, 2013) . The dominant soils in this area are clay with 46 slow water percolation and unique shrink-swell characteristics that are not favorable for 47 soil-based OWTS. An early study based on regional soil survey indicates that over 50% 48 of soils in the Alabama Black Belt are not suitable for traditional OWTS (He et al., 49 2011b). Unsuitable geology and soil challenges are further compounded by the 50 subtropical climate that during wet winter months increases the risk of saturated soil 51 overflow in Black Belt OWTS (Wedgworth and Brown, 2013) . 52

Most OWTS applications are based prescriptive design regulations that lack the 53 flexibility to cope with large environmental variations. In contrast, adaptive design 54 concepts are better able to cope with natural and societal variability to produce a more 55 sustainable, site-specific design. The benefits of adaptive design have long been 56 recognized by the irrigation industry and one particular case is soil moisture controlled 57 irrigation scheduling (Nikkels et al., 2019) . Soil moisture controlled irrigation scheduling 58 (SMCIS) holds promise to alleviate some of the geologic and climatic limitations faced 59 by OWTS in the Black Belt Prairie and similar soils by managing hydraulic loading to 60 disposal drain fields. Drain field soil moisture can be maintained within a relatively 61 narrow range to limit the extent of counterproductive soil shrink-swell, which can open 62 large fissures in the soil profile 2 m deep or more. More importantly, SMCIS in a drain 63 field can limit wastewater hydraulic disposal through an irrigation system during 64 unfavorable drain field conditions. This responsive wastewater application system can 65 create a cycle of alternating "flood/wet" and "drain/dry" phases that act as a passive 66 pump to expel and draw air into the soil to facilitate both aerobic and anoxic processes 67 (Zhi and Ji, 2014) . 68 irrigation (SDI) for wastewater dispersal/infiltration was field tested in the Black Belt 70

Prairie. The installed system proved effective as designed to prevent wastewater disposal 71 during unfavorable (i.e., saturated) drain field conditions (He et al., 2011a, 2013a). 72

However, several aspects of system performance were difficult to assess due to the 73 inherent limitations of the field study. For this reason, field study data was coupled with 74 numerical simulation to reveal some of the more complex processes within the system. 75

In this particular type of study, simulation through numerical models can be especially 76 helpful to enhance and further expand research findings (Henze et al., 2008) . 77

Process-based biokinetic models such as CW2D (Langergraber and Šimůnek, 2005) The single porosity van Genuchten-Mualem model was used as the soil hydraulic 120 property model, and the soil hydraulic parameters were obtained by the Rosetta method 121 embedded in HYDRUS using the field measured particle distribution over the 5 horizons 122 of the experimental site (Table S1) 3 Results and Discussion 143

The dynamic adaptation of the SMCIS-SDI wastewater application system 145 (hereafter referred to as system) during the cooler dormant season significantly curtailed 146 hydraulic application ( Figure 1 ). As expected, the system provided higher hydraulic 147 application rates during warmer growing season months characterized by higher soil 148 temperatures and relatively higher natural precipitations. Despite steadily increasing soil 149 temperature and ET after the winter (November 2007 through February 2008), the system 150 still required a period of months to recover desired, higher hydraulic application rates. 151

Observations demonstrate the effectiveness of the system to regulate hydraulic 152 application of wastewater. Although the system performs hydraulically as designed, the 153 relatively long period of curtailed hydraulic loading during the cooler dormant season 154 exposes a drawback in the system. The wastewater generated during the cool season will 155 need to be stored or directed to other means of wastewater treatment. Therefore, this 156 system appears to be inherently limited as a stand-alone sustainable wastewater 157 technology for rural sanitation in an area such as the Alabama Black Belt Prairie. In comparison to the control which was irrigated with clean water, the wastewater 234 application noticeably enhanced soil water NH 4 + -N and NO 3 --N. Despite early soil biofilm 235 development when the biological effect on NH 4 + -N and NO 3 --N was not adequately 236 established, the simulated trend generally conforms to field observations. The main 237 exception is field measured soil water NH 4 + -N is higher than simulated soil water NH 4 + -N 238 at 15 cm and 30 cm depths, which is likely caused by an overestimated nitrification rate 239 in the simulation. Even so, field measured soil water NO 3 --N demonstrates insufficient 240 denitrification of available NO 3 --N. Based on simulation, soil water NO 3 --N below drip 241 emitters is caused mainly by a lack of denitrification, while soil water NO 3 --N above drip 242 emitters results from upward capillary water movement. Soil water NO 3 --N accumulation 243 above drip emitters also suggest the potential for salt accumulation which was observed 244 in the field experiment ( Figure S4 and He et al., 2013a). 245 Table S2 (sand) and 290 S3 (clay). The result shows that higher emitter flow rates, a design parameter, provide 291

higher system hydraulic application rates. In a simulated sand profile the soil moisture 292 sensor position (i.e., the physical proximity of the soil sensor to the emitter) has a greater 293 impact on system hydraulic application rate than in simulated clay. In sand, under a given 294 emitter flow rate, the hydraulic application rate is increased as the distance from the soil 295 moisture sensor to the emitter increases, with horizontal location more influential than 296 vertical direction. This intuitive finding is due mainly to the influence of gravity on the 297 more highly permeable sand media. Similarly, in sand a longer system run time (on-time) 298 increases the impact of soil moisture sensor proximity on system hydraulic application 299 rate, but no such response is reflected in clay. This is caused by the different soil vertical 300 and horizontal movement potentials between sand and clay. 301

The placement of soil moisture sensors balances the need to minimize water 302 leaching loss and adequately hydrate soils for plant uptake. A larger horizontal distance 303 between the soil moisture sensor and the emitter will favor wetting soils between drip 304 laterals at the cost of greater leaching. A smaller horizontal distance between the soil 305 moisture sensor and the drip emitter tends to reduce water leaching, but may not 306 adequately wet all soil between drip laterals. This inverse relationship is more 307 pronounced in soils with higher permeability, indicating that soil permeability has a 308 profound impact on system hydraulic application rate based on quantifiable relationships between emitter flow rate, soil moisture sensor location, and system run time settings. 310

Even so, it should be emphasized that these simulations are made without considering the 311 influence from weather and crops, which will have noticeable impact on system 312 performance (Roberts et al., 2009) . 313 314 315 Figure S3 . Location of the soil moisture feedback positions used for simulation in Table S2 and   316   Table S3 . Simulated spatio-temporal water quality profiles during a typical 24-hour period are 341 illustrated for sand and clay in Figure. 4. For both media, readily biodegradable COD 342 (CR) is sufficiently removed within a limited distance around the drip emitter. In the 343 meantime, the spread of slowly biodegradable COD (CS) is larger than CR since CS 344 needs to be converted into CR before biological utilization. Even so, the differences 345 between CR and CS are wider in the sand than in the clay. Different from CR and CS, 346 biologically inert COD (CI) accumulation is found above the drip emitter in sand, but 347 below the drip emitter in clay. NH 4 + -N removal is nearly complete within a limited 348 distance from the drip emitter in both sand and clay. However, the apparent diffusion of 349 NH 4 + -N is wider in sand than in clay, which is due to the good permeability of sand. 350

The cumulative profile of NO 3 --N is found similar to CI in both sand and clay media. 351

The high permeability of sand favors leaching and also has the tendency to bring salts up The accumulation of CI and NO 3 --N also suggest the possibility of salt accumulation 361 which was noted during observations at the end of the one-year-long wastewater 362 application ( Figure S4 and He et al., 2013a). Since irrigation normally aims for water 363 conservation, the irrigation leaching requirement to flush soil salt accumulation is often 364 inadequately considered and implemented (Duan and Fedler, 2013). For soils with high 365 permeability, higher hydraulic application rates applied through SDI tend to bring salts 366 up to surface layers (Roberts et al., 2009 ). Likewise, the simulation demonstrates that 367 sand has a more pronounced surface accumulation of CI and NO 3 --N than clay. Typically, 368 a wastewater disposal system that is designed to maximize leaching applies wastewater 369 that contains more salts than required for agricultural irrigation. As a result, salt 370 accumulation should is more pronounced for wastewater disposal systems. 371

Soil salt accumulation is common in agricultural practices, and it is influenced by 372 weather conditions as well. Strong precipitation can leach accumulated salts from the soil 373 but at the cost of underground water quality (Raine et al., 2007) . On the other hand, a 374 high ET can be conducive to salt accumulation (Raine et al., 2007) . Therefore, salt 375 accumulation might be only a temporal issue for areas with changing season and weather 376 patterns. The subtropical climate of the Black Belt Prairie at times forces the system to 377 rest during any rainy season, and system resting can alleviate biofilm clogging as well as 378 leaching of accumulated salts from the drain field. However, this premise requires 379 additional field study for verification. 380 381

In an attempt to enhance the sustainability of rural sanitation, a SMCIS-SDI 383 wastewater disposal system was implemented in an experimental high clay site to adapt 384 system hydraulic disposal timing to drain field moisture conditions. After demonstrating 385 system effectiveness to prevent hydraulic loading during unfavorable drain field 386 conditions, a biokinetic process model CW2D was utilized to help evaluate the benefits 387 and limitations of the system in terms of environmental sustainability for rural sanitation. 388 System performance is found similar to other soil-based on-site wastewater 389 treatment systems in terms of nutrient management. Experimental and simulated results 390 converged to demonstrate that soil biofilm growth was promoted around drip emitters as 391 a natural response to wastewater application. Biofilm benefits include adequate removal 392 efficiency of applied biodegradable COD and NH 4 + -N. In short, biofilm formation is the 393 result of wastewater application, and the removal efficiency of COD and NH 4 + -N is the 394 result of biofilm formation. Nevertheless, the spatial distribution of biodegradable COD 395 and heterotrophs does not provide sufficient carbon to denitrify formed NO 3 --N, leading 396 to an excess of NO 3 --N with the potential for salt accumulation, as observed in the field 397 experiment. 398 Subsequent modeling of system performance in sand and clay media demonstrate 399 that the placement of soil moisture sensors in the drain field is sensitive to soil 400 permeability, and thus crucial for optimal system performance. With respect to hydraulic 401 and nutrient management, the soil moisture control strategy evaluated in this study is 402 demonstrated to be a viable technology that promotes the environment sustainability of 403 rural sanitation systems. 

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