First Flush Characteristics in Separate Sewer Stormwater and Implications for Treatment

sustainability Article First Flush Characteristics in Separate Sewer Stormwater and Implications for Treatment Sara Egemose * , Anne B. Petersen, Melanie J. Sønderup and Mogens R. Flindt Department of Biology, University of Southern Denmark, DK-5230 Odense M, Denmark; [email protected] (A.B.P.); [email protected] (M.J.S.); [email protected] (M.R.F.) * Correspondence: [email protected]; Tel.: +45-6550-7988 Received: 30 May 2020; Accepted: 18 June 2020; Published: 22 June 2020 Abstract: The characteristics of stormwater need focus due to climate change. Paved areas and recipients receiving more stormwater have an enhanced need for treatment before discharge to remove suspended solids, nutrients and xenobiotics. To handle and treat stormwater efficiently, knowledge concerning first flush patterns is crucial. Therefore, we have studied 12 entire rain events and correlated water volume, suspended solids, and dissolved and particulate phosphorus to catchment characteristics and precipitation characteristics. We have mainly studied smaller rain events (average 12.6 mm), as nearly 90% of the events in the studied catchment are <10 mm. We revealed first flush tendencies in 50% of the rain events, concerning both suspended solids and phosphorus. We also found significant correlations between catchment size and discharged masses, and most importantly between precipitation patterns and discharged mass per volume. A long dry period and low amount of rain during the previous event, as well as high rain intensity, gives a high chance of first flush. We conclude that stormwater treatment should focus on the initial part of the event, especially in areas with a lack of space and/or economy to handle and treat the entire event. Keywords: urban runoff; climate adaptation; pollution; retention 1. Introduction Climate change is among the many other effects causing more extreme weather, including cloud bursts, storms and droughts [1,2]. For the northern part of Europe, an increased amount of precipitation with more and heavier rain events is expected [3]. As an example, it is predicted that the number of 10 year events in Denmark will increase by 30% within the next 100 years [4]. Stormwater runoff from separate sewers is most often either discharged directly into the recipient or discharged after retention, e.g., in a wet stormwater pond [5]. Especially the first stormwater discharged will often have a larger content of particles and dissolved substances compared to the remaining part, as the first discharge contains all the accumulated particulate material from surfaces and the pipe systems [6,7]. This phenomenon is referred to as ‘first flush’, and is defined as a disproportionally high load of either concentration or mass in the first stormwater load during a rain event [7,8]. First flush depends on the dry period before the actual event, the intensity and duration of the rain event, the catchment type, and the transport distance of the discharge before the outlet (pipe length) [9]. Historically, different methods have been used to describe first flush events. A commonly used method is to express the percentage of mass transported along with a certain percentage of the volume [9], presented as normalized accumulated mass (L’) for each time as a function of normalized accumulated volume (V’) at each time [9]. A 45◦ bisector will then represent a proportional transport of mass and volume, see example in Figure1. If the curve is above the bisector, a relatively larger mass is transported in the discharge compared to the transported volume, whereas Sustainability 2020, 12, 5063; doi:10.3390/su12125063 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 5063 2 of 9 Sustainability 2020, 12, x FOR PEER REVIEW 2 of 9 the opposite is the case if the curve is below the bisector; in that case, the runoff is diluted. So, if first whereas the opposite is the case if the curve is below the bisector; in that case, the runoff is diluted. flush is occurring, the curve will be above the 45 line [9,10]. So, if first flush is occurring, the curve will be above◦ the 45° line [9,10]. Figure 1. Schematic L’/V’ curve—redrawn after [9]. L’ is the mass load and V’ is the volume. Both are Figure 1. Schematic L’/V’ curve — redrawn after [9]. L’ is the mass load and V’ is the volume. expressed as normalized accumulated values. Both are expressed as normalized accumulated values. TheThe numerical numerical definition definition of of the the first first flush flush varies. Some Some have have suggested suggested that that first first flush flush is is characterizedcharacterized by by an an 80% 80% mass mass transport transport during during the the first first 30% 30% of of the the volume—a volume—a 30/80 30/80 first first flush flush [10,11]. [10,11]. OthersOthers havehave arguedargued for for an evenan even stronger stronger characterization, characterization, where where 80% of the80% mass of the should mass be transportedshould be transportedduring thefirst during 20% the of thefirst volume—a 20% of the 20 volume—a/80 first flush 20/80 [7]. Finally,first flush some [7]. studies Finally, have some suggested studies ahave less suggestedstrict definition a less withstrict first definition flush at with 20/40 first or 25flush/50 asat 20/40 first volume or 25/50/mass as first [9]. volume/mass In general, first [9]. flush In general, is most firstcommon flush inis smallermost common catchments in smaller (<10 ha)catchments [11]. (< 10 ha) [11]. AA well-known well-known method method to to store store stormwater stormwater and and avoi avoidd hydraulic hydraulic effects effects on on the the recipient, recipient, as as well well asas to to remove remove primarily particulate substances, substances, is is wet stormwater ponds [12,13]. [12,13]. In In Denmark, Denmark, wet wet pondsponds are are often often designed designed with with a a controlled controlled outlet outlet in intensitytensity of of 1–2 1–2 L/s/ha L/s/ha catchment, catchment, a a permanent permanent wet wet 3 volumevolume of of 150–250 150–250 m m3/reduced/reduced hectare hectare and and a a storage storage volume, volume, if if properly properly designed designed [5]. [5]. The The reduced reduced areaarea is thethe impermeable impermeable proportion proportion of theof catchment.the catchment. Many Many of the of wet the stormwater wet stormwater ponds are ponds designed are designedwith an overflow with an facilityoverflow placed facility near placed the pond near outlet, the whichpond outlet, is activated which if theis activated pond’s storage if the capacitypond’s storageis exceeded. capacity The is result exceeded. is that The it will result most is likelythat it be will the most discharge likely that be the entered discharge the pond that first entered that willthe pondbe discharged first that firstwill duringbe discharged overflow. first Put during another overflow. way, the Put first another flush holdingway, the the first most flush polluted holding water the mostmight polluted be discharged water might through be discha the overflowrged through if the storagethe overflow capacity if the of storage the pond capacity is exceeded, of the pond and the is exceeded,cleaner water and the coming cleaner after water the firstcoming flush after is keptthe first in the flush pond is kept and in treated. the pond That and is treated. an undesirable, That is anand undesirable, environmentally and environmentall and economicallyy and ine economicallyfficient way ofinefficient handling way stormwater. of handling stormwater. Therefore,Therefore, aa greatergreater knowledge knowledge of theof the first first flush flush phenomenon phenomenon will support will support the knowledge the knowledge platform platformso that the so designthat the of design stormwater of stormwater ponds can ponds be optimized can be optimized and make and it possible make it topossible retain andto retain treat and first treatflush, first without flush, making without the making ponds the inappropriately ponds inappr big.opriately Barbosa big. and Barbosa Hvitved-Jacobsen and Hvitved-Jacobsen have suggested have suggestedthat the treatment that the treatment efficiency efficiency could be enhancedcould be enhanced by placing by the placing overflow the overflow facility at facility the inlet at the instead, inlet instead,so that theso that more the diluted, more diluted, cleaner stormwatercleaner stormwat cominger aftercoming first after flush first is bypassedflush is bypassed instead of instead the most of thepolluted most firstpolluted flush [first6]. Thisflush will [6]. not This change will thenot hydraulic change the load, hydraulic but it will load, entail but that it ifwill overflow entail that occurs, if overflowit will be occurs, the cleanest it will water be the that cleanest overflows. water A that model overflows. study in A progress model study made in on progress Danish ponds made hason Danishrevealed ponds that thishas revealed change in that design this canchange effectively in design increase can effectively the treatment increase effi ciency.the treatment efficiency. TheThe important important question question is is what what proportion proportion of of th thee first first flush flush should should be be kept kept in in the the stormwater stormwater pondpond and treated. ToTo provideprovide more more information information about about that, that, we we have have examined examined the the runo runoffff from from 12 rain 12 rainevents events in three in three different different catchments catchments with varying with va intensity,rying intensity, duration duration and dry and weather dry weather periods sinceperiods last sincerain event.last rain The event. aims The were aims (1) towere characterize (1) to characterize to what extent to what first extent flush first was flush observed was withobserved respect with to respect to suspended solid (SS), dissolved phosphorus (DP) and particulate phosphorus (PP), and (2) to evaluate which catchment and precipitation patterns the first flush was dependent on.

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