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  • Writer's pictureHydroPhos Team

What Do We Do With All Of This Wastewater?

Over the last few articles, HydroPhos has discussed the dangers of phosphorus runoff, which leads to dangerous algal bloom interfering with local ecosystems. If something is to be done to mitigate and prevent this, the source points need to be identified and targeted. Broadly, the source of most of this phosphorus runoff is wastewater, but wastewater itself is a broad term arising from a variety of processes, which is typically treated before discharging to some natural body of water.[1] [2] Since approximately half of eutrophication originates in wastewater, mitigating eutrophication will depend upon a firm understanding of wastewater.


Wastewater treatment plants (WWTPs) target the removal of contaminants specific to the type of wastewater. This article will break down the main four types of WWTPs used in North America and Europe.


Wastewater is any liquid stream of undesired quality produced during some process. This can be from:

○      Municipal waste, such as residential plumbing and city sewers,

○      Industrial processes that include everything from food and beverage to chemicals to oil and gas,

○      Agricultural byproducts, that may arise from animal fecal or biologic matter produced in confined animal feedlots

○      And landfill leachate, which is the water that has mixed with landfills from rainfall or breakdown of solid waste.


Some industries and particular industrial plants will have their own WWTPs, but other municipalities permit local plants to discharge their wastewater with the municipal wastewater. This makes municipal wastewater, or sewage water, treatment plants the most prevalent WWTPs, with over 16,000 such plants across the United States.


Each WWTP needs to be tailored to the specific contaminants found in the wastewater. For example, some industrial processes produce large amounts of waste acid that needs to be neutralized before discharge. But each system generally consists of multiple stages to tackle the multiple different problems in each wastewater.


Stages of Municipal Wastewater Treatment Plants


Municipal waste can come either from residential sewage as well as storm runoff. Municipalities often have complex systems of storm drains, but the runoff during storms can wash away nutrients and sediments from lawns and gardens, including the fertilizers on them. This wastewater is directed towards municipal WWTPs, which utilize a series of stages to treat different contaminants in the water.


The aim of primary treatment is to remove the bulk of the organic and solid matter. First, a screen is used to remove large objects and floating debris. The water then passes through a sedimentation tank, where heavier solids fall to the bottom, while the effluent water exits into secondary treatment.



Secondary treatment targets as much biological and organic matter as possible, typically utilizing aerobic digestion, in which bacteria in the aeration tank consume the organic material. Finally, chlorine is typically dosed as a disinfection step to kill this bacteria before discharge. Many governing bodies added a need for dechlorination to remove this excess chlorine.


Many WWTPs end at this point, with effluent safe to discharge into the local waterway. The rise in novel and/or larger concentrations of micropollutants has led to the concomitant rise in WWTPs with a tertiary step. This is only further exacerbated with nutrient waste and the regulations to combat them.


The type of treatment in this step is determined by that which must be removed. Tertiary treatment can include sand filtration, particle filtration, activated carbon filtration, membrane separations, membrane bioreactors, etc. Most of these processes are size-selective separation processes, designed to remove particulate matter based on their size. If heavy metals need to be removed, nanofiltration can be used. If toxins seep through secondary treatment, then the water stream can be passed through activated carbon.


Industrial WWTPs


Industrial wastewater treatment plants are highly specific to the targeted industry. For example, a chemical processing facility may utilize ion exchange columns or neutralization steps followed by filtration since the primary waste may contain little biologics but large concentrations of metals or acids/bases. Food waste may be of greater issue in food and beverage industries, where cartridge filtration and membranes are better utilized.


Sometimes industrial wastewater is combined with the sewage waste, where local regulations allow for direct dumping. Other times, this waste is stored in lagoons until it can be hauled away from site to be specially treated. In this latter case, lagoons can often overflow due to excessive precipitation. This will cause the wastewater to run into storm drains that feed municipal sewage treatment centers ill-equipped to handle this specialized waste.


Agricultural WWTPs


Agricultural WWTPs primarily focus on animal fecal matter, often using anaerobic lagoons, where animal waste is stored in water until it can be applied later to fields. Soil can only incorporate so much waste matter. But these lagoons cannot deal with a large source of wastewater from the agricultural industry: nutrient and sediment runoff. This occurs out in fields and cannot be easily directed towards a central treatment plant.


Landfill Leachate Treatment


Finally, great strides have been made in linings and prevention of this leachate entering the groundwater. But now this leachate must be regularly treated and safely discharged. Filters, sumps, and membrane separations are utilized to prevent toxic metals from entering sewage systems, but scaling and fouling limits the effectiveness of reverse osmosis elements. Chemical dosing is also often used to neutralize the acidic leachate and precipitate some potential scalants.


Conclusions


Regardless of where the wastewater originates, all of it will end up in the ecosystem, in one waterway or another, in surface water or groundwater, better treated or untreated. And excess nutrients, like phosphorus and nitrogen, can come from any of them. Phosphorus from detergents and garden fertilizers enter sewage WWTPs, while agricultural waste is a large source of the phosphorus runoff. Nature can usually absorb some of this, but not at these high concentrations. Much of this ends up in storm runoff entering bodies of water directly or sewage treatment.


A key to removing this phosphorus will be in reducing the load on the ecosystem by adding a step to WWTPs. This is currently done but with limitations. More cost effective and easy-to-implement options will need to be employed. HydroPhos is committed to this treatment and removal of phosphorus, making it more cost competitive by harvesting and reusing the waste phosphorus.

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