Constructed Wetland Design: Types, Function, and Treatment Processes

[00:00:01]welcome to wetland design so when we're talking about wetland design we're talking about constructing a wetland or creating a wetland where one would not otherwise exist so when we design a wetland we're creating a wetland um in a place that wasn't one before you actually when we talked about section 404 of the clean water act with creating mitigating all wetlands you can't use an existing wetland so this has to go in a place where there was not a wetland it can be used to treat wastewater it can be used for an ecological function there's many reasons why you would design a wetland um but it's created to meet some sort of ecological or human need there's many types of constructed wetlands the basic ones are a surface flow wetland or you also hear them described as free water surface so fws and basically as that means it means the water is you can see the water uh it's free water surface um a subsurface flow is um also called a rock reed system where basically you do not see the water the water is flowing underneath the surface of the wetland and the plants do emerge out but it flows through a rock subsurface and often you'll have reed plants that are then emerging out there's also storm water wetlands so these are wetlands that are designed to hold water and then infiltrate it slowly and then we have vertical flow systems these are a type of subsurface flow but instead of it flowing just kind of through verti horizontally through the system these are going to flow vertically down through the system so it creates oxygenated and anoxic or without oxygen zones so we start with our subsurface flow wetland again it's an emergent macrophyte treatment system so we have plants that are doing a lot of the treatment um and microbes and where again the water is coming onto the surface and then it's leaving via surface flow so the first thing when you construct a subsurface wetland is you're going to need some sort of liner so the liner can be a clay liner but you need to again if it's a clay liner you need to ensure that the liner is kept wet because you don't want it drying out and cracking in the clay or you can use a plastic or geo membrane liner so again especially if you're putting in waste water you're treating it that you don't get infiltration now you may not want a liner you may actually if you're doing a surface wetland for ecological functions we may want the water to infiltrate but again if we're doing some sort of treatment system then we might need a liner in there um commonly to build these systems the levees are basically pushed up from the side and the volume is partly dug out and um and again you might create some mounds or some undulations in the system so that you have variations in the water and so that you have emergent vegetation and maybe some standing vegetation as well so the hydraulic loading the water coming into the wetland can be controlled using an orifice where you basically have a tube and you drill a hole in the middle and um so it can come through a point flow or you can build a manifold so manifold is going to be going across and then you're going to have holes that going across the manifold so instead of it coming in one point of the wetland it might go across the whole you know and horizontal flow through the wetland across a horizontal distance um in terms of the um outflow um the level of the outflow so the bottom of this pipe is going to control how much volume is in your wetland and so for your outflow you may actually have some some river walks to protect from um uh uh um roots getting in and you might actually put like a cheesecloth even so that you don't have roots coming into your pipes um but basically you're going to have the water coming into this pipe and and then again the elevation that's going to come up here so it's not going to discharge here otherwise your water will just flow right through you know so it's not like a bathtub it actually has to come here where it's coming in it's flowing through and then it's going to come out based on the height of this pipe um the emergent wetland plants fill a variety of roles they promote microbial degradation of the waste they also uptake nutrients and contaminates the plants to themselves they also create microsites within the root zones so there's specialized structures in wetland plants called aranchyma and basically it allows oxygen to reach into the rhizophore and so these plant stems they can provide not only for oxygen to leach down into the root zones again minimal oxygen these are still anaerobic zones but it creates these aerobic microsites um for the oxygen to reach these root zones as well as the stems provide colonization in the for um plants um the actual as you build these emergent plants um the stems can actually help promote uniform flow so you don't have preferential flow patterns through your wetlands they can also shade the water which is actually good because we don't want a lot of algal growth and phytoplankton growth because that can be a source of additional organic matter additional body because when it degrades then you're going to have more organic matter coming into your system they can also inhibit wind mixing and again that wind mixing is going to create less uniform flow and maybe create a preferential flow pattern and so having a lot of emergent vegetation is going to help so if we think about the plants that you have you might have duckweed and other floating aquatics you're going to have the emergent plants as well and then you're going to have detritus from the plants and so within this you're going to have your particular body your organic matter you're going to have organic based phosphorus and nitrogen in the detritus and settled total suspended solids you might have metals and other things that are suspended here again you're going to have low dissolved oxygen that's going to be anoxic and you're going to want this to be as vegetated as possible again you're going to remove through flocculation sedimentation absorption and anaerobic reactions are going to be your primary removal mechanisms you're going to get some um removal through some oxygenation through atmospheric reaction at the surface not a lot but some and then again some oxygen oxygen coming from the the roots of special wetlands plants you're going to get some uv radiation which might help with decreasing coliforms and then you also might have submerged vegetation in addition to emergent vegetation and then you might even have an open water zone and in that open water you're going to get more pathogen kill and you might get a little bit of atmospheric aeration to get um some aerobic treatment the advantages again you can see the water and so it's really good for outreach it's good for recreation walking trails birding it actually creates habitat in terms of wildlife habitat increasing biodiversity and nursery for aquatic species and it can provide other services like flood control and storm surge because the water is at the surface and you know depending on how it's built it may be able to hold some of that water in addition to other um benefits that it has potential risks again with surface flow wetlands we are worried about creating mosquito breeding grounds and risk of west nile virus malaria and other diseases but there are things you can do to try to help control mosquitoes um you can actually kind of build it with street with steep concrete slopes with deeper bottoms and you can also put um mosquito fish that actually gambuza that actually can eat some of the larvae and you can also do non-mosquito um conducive plants so for example like floating lettuce tints actually have um little pockets of water in there and the mosquitoes can breathe in there so you want to think about what type of plants you have both emergent and submergent and then also there are mosquito-specific bacteria that can eat that can break down the larvae in terms of subsurface wetland here we're not going to see the water at all so we're not going to get a lot of the um storm water surge control we're not going to get a lot of as many wildlife habitat benefits because all the water is actually going to be below the surface so we're going to dig out here we're going to plant our plants and we're going to have gravel through this entire media and we're going to have our plant roots within that gravel and then it's going to move horizontally through and so here our height is controlled by the height of this pipe so it's actually gonna our pipes gonna be up here and then it's gonna come up and kind of spill over and that'll be the height of your water so it's filled with again river stones or gravel you don't want to use fine sand because you want actually void spaces the treatment volume is actually you want the water to be able to move through and you want a lot of water to be there so the treatment volume is actually the space between the stones um so you design for that void volume because that's where your that's how much water can go through because the stones are going to take up space so it's the void volume is the space for your water to go through but the stones are actually really good in terms of providing surface area colonizing surface for microorganisms as well as you have those root zones as well which an important part of treatment both in subsurface and surface wetlands and so um you might have for example this was a wastewater system you might have a septic tank and then which you'll get a lot of settling of solids because you don't want for subsurface wetlands um you don't want a lot of solids going into your system so if you have a lot of solids it would be better to have a free surface wetland where you can actually have the solids settle out in the zone because if there's a lot of solids going through your subsurface wetland they're going to clog and then you won't have that void volume that's so important um so you might actually have a settling tank and then you would have your subsurface here again especially when we're treating wastewater we may want it to be subsurface for environmental protection um and so that we don't have coliforms introduced into the environment and you may have it where some of these are are zero discharge or you may have other discharge and then you could even then after that put into your leach field and have infiltration um if you see water and the surface of a subsurface wetland then that's a good indication of plugging or some clogging or an overload problem um you can actually step feed these systems so your wastewater inflow you can have it at more than one point so here we have a step feeding system where we have actually waste water coming in here also coming in here and also coming in here and so that we can make sure that we get a more uniform flow as we go through and utilize more of the surface area the length times the width by just spending it along a greater portion of the wetland instead of just having it come here you can have it come in various steps which can avoid kind of clogging a media of having one input and could have some better aeration so if we have a subsurface with vertical flow instead of the water coming in here and then exiting out at the top excuse me exiting out at the top if we have it with vertical flow then it's actually going to exit out of the bottom so it actually comes in so you actually have an aerobic zone so water isn't filled all the way at the top it actually has an aerobic zone and then it needs and the water is actually only in this bottom part and so then you have your anaerobic zone and then you have um your discharge at a lower elevation so it actually is it's less time that it's in the system but it has both an aerobic and anaerobic um point but there's less treatment volume there um so storm water wetlands are going to differ from surface and subsurface wetland and that the loading isn't uniform or continuous so it's not something that's being pumped into every day it's not something we're necessarily using um at the back of a wastewater treatment or a back of the septic system so these are going to be shallow detention ponds that are kind of use that first flush for treating runoff so they may be dry during parts of the year but then when they do have kind of that first flush which has a lot of contaminants and other things that can go into the wetland fill that up and treat it during that um during that time so if we think about a um a hydrograph if we think about the flow um the cubic feet per second again we're not going to have a lot of rain and then all of a sudden it comes and we just have this huge flow come out so if we're able to route that to a wetland or a wetland pond the idea is that it'll slowly fill up and then it'll slowly over um hours let that water come through versus a conveyance pipe which again can cause a lot of erosion and damage to our streams so the idea is that we um we figure out this design storm and how big we need to make our wetlands so that a large magnitude of it can then settle into our wetland and infiltrate and slowly discharge so there's a lot of again a lot of wetlands are um are built for municipal treatment but not always i'm actually one of the biggest builder of wetlands is actually ducks unlimited more for habitat but again a lot of municipalities do build wetlands for municipal wastewater and so this would be the tertiary treatment or sometimes even the secondary um but usually it's after settling of solids and then it would go through a wetland um so in this manual it has everything you need to know about designing them and about the equations that you need to use um but i did want to point out some of these myths that are in this manual and so basically they're saying that these are um things that we need to consider when we're designing wetlands because there's not always um information out there that that that it makes it easy to design and the myth number one is that wetland design is easy to do with published equations so yes we're going to go over published equations and there are published equations but you need to be careful and one of the reasons is whenever we're studying wetlands i may get a influent point on you know september 1st my effluent point that i also get on september 1st is not the same as that influence wastewater so if we had a big plug of pollution that came through that's going to show up in my effluent for example but if it already went through or it's already in the wetland it's not going to show off my influence so it's hard to pair those data and even if i know that i've about a two week retention time and so i take samples every two weeks with the idea that my effluent from two weeks later can match with my influence we don't really know the exact retention time and not all those particles have the exact same retention time um and there's also rain especially if it's a surface wetland how much rain happened between those two weeks was their solution from that rainwater and so it's really hard to pair up these effluent and influent data with these grab samples that we might take um and it's hard to get that valuable retention time data also often with these equations things like temperature precipitation aren't included and so um the other thing we need to worry about is often cleaner wastewater effluents so we may be putting wastewater in with only one milligram per liter nitrogen and then i'm using the same question when i have 100 milligrams per liter of nitrogen or 20 milligrams per liter nitrogen and we may not getting be getting those same removal rates with that a higher concentration so again the misconception is that wetland design's been really well characterized by published design questions but in reality they're complex systems um in terms of biology hydraulics and water chemistry and there's a lack of quality data of sufficient detail both temporarily and spatially on full-scale constructed wetlands due to this lack of data we've been forced to design you know derived design parameters by aggregating performance data from a variety of wetlands which can lead to uncertainty about the validity of these parameters um the myth too is that they have ample aerobic and anaerobic zones and while plants can get through for metaphors and um and through these kind of like straws in their stems and they can get plant oxygen to their to their roots and some is released into the environment but usually the oxygen demand of this water overwhelms the small addition of oxygen to the water column through all the detritus that is there the algae as well as if you're putting waste water in so the misconception is that they have ample that they have aerobic as well as anaerobic treatment zones um it's probably the most common misconception is the ability this emergent wetlands to transfer auction to the roots and so emergent plants are uniquely suited to the anaerobic environmental wetlands because they can move oxygen from the atmosphere to the roots and research has shown that some of this oxygen leak from the roots goes to the surrounding soil so this phenomenon and early work with national constructive wetlands that treated um wastewater with a low oxygen demand has led to the assumption that there's significant anaerobe aerobic microsites within all wetland systems but again it this doesn't always happen within all systems so we need to be careful about the type of waste that comes in so if we know we're getting a lot of ammonia coming in then we may not get ample treatment because we need aerobic zones for nitrification to happen before denitrification can happen in our ample anaerobic zones and that goes to the next one that they can remove significant amounts of nitrogen so again the misconception is that they can remove this nitrogen related to this misconception about the availability of oxygen and constructed wetlands and therefore their ability to remove um nitrogen harvesting removes about 20 percent of the influent nitrogen studies have shown at conventional loading rates which release nitrification denitrification which is how we remove nitrogen and wastewater treatment plants as the primary removal mechanisms so again if ammonia is uh if we aren't going to get nitrification of ammonia unless we have ample open water vertical zones or other designed oxygen inputs otherwise they may be used in construction with other aerobic treatment track practices that can nitrify so you may have a trickling filter first and then go into your subsurface wetland and myth number four is that they can remove significant amounts of phosphorus phosphorus removal is limited to seasonal uptake by plants which is not only um minor compared to the phosphorous load and a lot of municipal treatment but it's also negated by again these plants um you know they're they're gonna um they're gonna uh unless they're harvested they might um their leaves fall down that phosphorus is still in the leaves then and that falls down into the water um again you may get some absorption but this absorption is limited in terms of how many absorption sites that we have so um in many tertiary wetland plants with low in fluid concentrations the removal of one milligram of p is report is a large percent removal but that's not that much and so when we looked at newly re um constructed wetlands we also get a lot of phosphorus removal due to a lot of microscience there for phosphor absorption and new plants are going to take up more phosphorus than mature wetlands and so we need to just be be clear about what what kind of phosphorus absorption capacity we're going to have over time doesn't mean it doesn't happen we just need to plan for it so when we do design wetlands we're looking at a hydraulic budget first and so we're looking at our q1 which is our inflow and our concentration of whatever it is the organic matter the nitrogen the phosphorus whatever's coming in we're going to have the flow and the concentration we're going to think about evaporation and precipitation and then we have our area of our wetland and then we'll think about our outflow the rate of outflow and the concentration in that outflow so when we just think about hydrology we're going to have our q in minus our q o plus precipitation minus evaporation is going to be our equation that we're going to use um which is it which is a reaction of the volume of water and time again in this example groundwater infiltration is excluded because this is assuming there's a liner if there's not a liner then you also need to account for groundwater infiltration your hydraulic budget um so for evapotranspiration we're going to use this equation that again is a function of temperature the daily temperature and the number of days and it's going to vary month to month in terms of how much evaporation that we get if we're talking about organic removal again we want that loading to be distributed over a significant portion and not at a single point if we can um and again the depth should be about 24 inches or less to assume that we are going to have some if we have a lot of organic matter coming in so that we do get some some aerobic just um um aerobic um oxygen oxygenation um we might even need to do partial recirculation in the summer months to overcome the evapotranspiration loss um to maintain design flow rates um again we can control it by that step feed distribution again having it various throughout or recycling of the discharge a loading rate of about 112 kilograms bod per hectare per day um is a typical upper lowering rate that we don't want to go over and again that oxygen we can transfer about five to ten grams per day it's not a lot per meter squared of um per meter for surface area meter squared of wetland surface and so it can be lower depending on the vegetation cover that we have so organic matter body removal in a wetland has been described by a forced order model so we have our concentration in the influence of vod divided by our concentration and the effluent excuse me divided by our concentration in the insulin what's coming in equals an exponent of again e to the negative k times t both of those are up in the exponent so e taken to the negative k times t with our negative k being the temperature dependent for shorter reaction rate that's our k that's our rate and then the t being the hydraulic retention time in days and so our reaction rate is based on the temperature so our kt you basically have the reaction rate that you would get from a book for whatever we're looking at the concentration that we're looking at um and then you have your reaction rate that you're going to get at 20 and then you're going to do a temperature rate constant so that you can figure out well at 20 degrees celsius this is the reaction rate i get but in the summer i'm going to have this reaction rate and in the winter i'm going to have this reaction rate and so your reaction rate's going to change based on the actual temperature conditions we also need to calculate the area needed and so our area again is going to be based on the flow times again the natural log because before it was e the opposite of our x our e is our natural log so we're going to have the flow times the natural log of the concentration coming in minus the concentration going out divided by the k dod and again it being temperature dependent um so for this example for example it's 0.1 at 20 degrees but you would adjust that based on your degrees um to figure out your hydraulic retention time that's going to be your length times your width times your depth divided by your flow rate okay so it's it's you know meters of length meters of width meters of depth divided by the um the cubic foot per meter cube that's coming in for example the amount of water coming in for retention time for a subsurface wetland though it's going to be a function of the porosity which is the remaining volume of the cross-sectional area so the volumes is the void volume and the volume are the voids that are there so depending on the type of media that you have your porosity is going to be different so with gravelly sand we're going to have much less porosity and less void volume than if we have medium or coarse sand and so that's going to affect both our hydraulic conductivity how it moves through the system as well as our reaction rate our k value so we can combine these equations to get our temperature times our depth times our area times our porosity divided by our um flow rate to get our detention and the retention time in the wetland um again we can also look at the slope again depending on if we have a slope with our wetland if it's a vertical flow wetland then we need to take into account that as well and then you can combine them all to get this really big equation and that basically tells us our concentration so if we know our concentration coming in we can then calculate our concentration coming out using this equation area kt area 1.75 length times width times depth times ferocity divided by flow all right thank you you