Indicator and Pathogen Removal by Low Impact Development Best Management Practices
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water Review Indicator and Pathogen Removal by Low Impact Development Best Management Practices Jian Peng 1,*, Yiping Cao 2,*, Megan A. Rippy 3, A. R. M. Nabiul Afrooz 4 and Stanley B. Grant 3 1 Orange County Environmental Resources, Orange, CA 92865, USA 2 Southern California Coastal Water Research Project, Costa Mesa, CA 92626, USA 3 Department of Civil and Environmental Engineering, University of California, Irvine, CA 92697, USA; [email protected] (M.A.R.); [email protected] (S.B.G.) 4 Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, USA; [email protected] * Correspondence: [email protected] (J.P.); [email protected] (Y.C.); Tel.: +1-714-955-0650 (J.P.); +1-714-755-3241 (Y.C.) Academic Editor: Miklas Scholz Received: 25 October 2016; Accepted: 10 December 2016; Published: 16 December 2016 Abstract: Microbial contamination in urban stormwater is one of the most widespread and challenging water quality issues in developed countries. Low impact development (LID) best management practices (BMPs) restore pre-urban hydrology by treating and/or harvesting urban runoff and stormwater, and can be designed to remove many contaminants including pathogens. One particular type of LID BMP, stormwater biofilters (i.e., vegetated media filters, also known as bioinfiltration, bioretention, or rain gardens), is becoming increasingly popular in urban environments due to its multiple co-benefits (e.g., improved hydrology, water quality, local climate and aesthetics). However, increased understanding of the factors influencing microbial removal in biofilters is needed to effectively design and implement biofilters for microbial water quality improvement. This paper aims to provide a holistic view of microbial removal in biofilter systems, and reviews the effects of various design choices such as filter media, vegetation, infauna, submerged zones, and hydraulic retention time on microbial removal. Limitations in current knowledge and recommendations for future research are also discussed. Keywords: biofilter; recreational water quality; indicator bacteria; pathogen; stormwater; urban runoff 1. Introduction Urbanization, with concomitant increase in impervious surfaces, results in increased volume and rate of stormwater flow; reduces natural infiltration of stormwater; negatively impacts stream and coastal ecosystems; and often carries significant pollutant loads, including pathogens, into receiving waters [1], which provide significant values both as habitat and a recreational resource. Every year, millions of people recreate at beaches, creeks, lakes, and other water features, generating billions of dollars in economic revenue [2]. However, in the United States, microbial-contamination of recreational waters is one of the top causes of surface water quality impairment [3,4], despite decades of investigation, rule-making, and management efforts across the nation. Stormwater reuse is also becoming an increasingly attractive resource management strategy [5]. Yet, microbial contamination, among all stormwater contaminants, is the most problematic for water reuse [6]. Low impact development (LID), a planning and environmental management practice that focuses on restoring the hydrology of an urbanized watershed to its pre-development condition, has been increasingly used to improve human water security (e.g., by providing a ‘fit-for-purpose’ source Water 2016, 8, 600; doi:10.3390/w8120600 www.mdpi.com/journal/water Water 2016, 8, 600 2 of 24 of water), improve receiving water quality, and mitigate hydrological factors that contribute to the urban stream syndrome [1,5], including the symptoms associated with poor microbial water qualityWater (e.g., 2016, [87, 600–9]). LID best management practices (BMPs) include infiltration BMPs (bioretention,2 of 23 bioinfiltration swales, rain gardens, collectively called ‘biofilters’), pervious surfaces (porous quality (e.g., [7–9]). LID best management practices (BMPs) include infiltration BMPs (bioretention, pavements, concrete, or asphalt), dry wells, detention ponds, and proprietary systems. Among these, bioinfiltration swales, rain gardens, collectively called ‘biofilters’), pervious surfaces (porous biofilterpavements, systems concrete, are increasingly or asphalt), common dry wells, features detention of the ponds, urban and landscape proprietary and systems. are often Among prioritized in LIDthese, implementation biofilter systems due are to increasingly their multiple common benefits, features including of the filtration, urban landscape infiltration/groundwater and are often recharge,prioritized evapotranspiration, in LID implementation urban heat-island due to cooling, their andmultiple aesthetics benefits, [5]. including filtration, infiltration/groundwaterTypical biofilters are below-grade recharge, evapotranspiration, areas filled with urban a designed heat‐island mix cooling, of soil and media aesthetics (sand, [5]. mulch, loam, etc.),Typical vegetated, biofilters and are underlain below‐grade by areas sand filled or gravel with a withdesigned an underdrainmix of soil media and (sand, an overflow mulch, pipe (Figureloam,1). Theetc.), bottom vegetated, of theand biofilter underlain system by sand may or begravel pervious with an or underdrain impervious. and Depending an overflow on pipe the flow balance(Figure and 1). the The elevation bottom of of the the biofilter underdrain system and may overflow be pervious pipe, or thereimpervious. may be Depending a submerged on the zone flow at the bottombalance that couldand the provide elevation additional of the underdrain pollutant and removal overflow [9, 10pipe,]. If there the stormwater may be a submerged flow is large zone enough, at the bottom that could provide additional pollutant removal [9,10]. If the stormwater flow is large a layer of standing water may accumulate in a surficial ponding zone. Excess water can be released by enough, a layer of standing water may accumulate in a surficial ponding zone. Excess water can be an overflowreleased drainby an overflow to avoid drain flooding. to avoid flooding. Figure 1. Schematics of a typical stormwater biofilter with (B) and without (A) a submerged zone Figure 1. B A (SZ). BothSchematics biofilters of are a typical planted stormwater with a variety biofilter of plant with types ( reflecting) and without different ( ) common a submerged above zone and (SZ). Bothbelow biofilters ground are morphologies. planted with These a variety include of plant herbaceous types reflectingmonocots with different branching common roots above and shallow and below ground(1) vs. morphologies. moderate (3) Theserooted includedepths, as herbaceous well as upright monocots (2) and with creeping branching (4) dicots roots with and deep shallow rooted (1) vs. moderatedepths, (3) low rooted root depths,branching, as and well high as upright vs. low specific (2) and root creeping density. (4) dicots with deep rooted depths, low root branching, and high vs. low specific root density. Water 2016, 8, 600 3 of 24 Water 2016, 8, 600 3 of 23 Since one major benefit of biofilters is water quality improvement, there has been great interest in Since one major benefit of biofilters is water quality improvement, there has been great interest studyingin studying the mechanism the mechanism and efficiency and efficiency of biofilter-mediated of biofilter‐mediated removal ofremoval stormwater of stormwater contaminants. Grebelcontaminants. et al. [11] proposed Grebel et three al. [11] removal proposed mechanism-based three removal mechanism strategies‐ tobased increase strategies removal to increase of common stormwaterremoval contaminants,of common stormwater including contaminants, microbial contaminants,including microbial by engineeredcontaminants, infiltration by engineered systems, includinginfiltration biofilters. systems, These including strategies biofilters. include These choice strategies of infiltration include media,choice of manipulation infiltration media, of system hydraulicmanipulation behavior, of andsystem manipulation hydraulic behavior, of redox and conditions. manipulation Rippy of [12 redox] summarized conditions. and Rippy conducted [12] a meta-analysissummarized to examineand conducted relationships a meta between‐analysis variousto examine design relationships choices and between removal various of fecal design indicator bacteria.choices Jiang and et removal al. [13 ]of focused fecal indicator on microbial bacteria. risk Jiang assessment et al. [13] focused of stormwater on microbial harvesting risk assessment by various of stormwater harvesting by various LID, including biofilters. This review aims to build upon LID, including biofilters. This review aims to build upon previous reports to provide a holistic view previous reports to provide a holistic view (Figure 2) of factors affecting the removal efficiency of (Figure2) of factors affecting the removal efficiency of microbial contaminants by biofilters. Particular microbial contaminants by biofilters. Particular attention is paid to biofilter design considerations attentionand isthe paid removal to biofilter of indicators design considerations(i.e., the current and water the removalquality compliance of indicators requirement) (i.e., the current versus water qualitypathogens compliance (i.e., the requirement) actual agents versus of public pathogens health risk). (i.e., the actual agents of public health risk). Figure 2. Conceptual model for removal of indicators and pathogens by biofilters. Spore‐forming Figure 2. Conceptual model for removal of