Crash Course

Sept. 7, 2018

Learn the basics of wastewater treatment, water reuse & water recycling

About the author:

Madan Arora, Ph.D., P.E., BCEE, is technical director for Parsons Corp. Arora can be reached at [email protected].

Did you know wastewater is 99.9% to 99.95% water? It is only the remaining 0.05% to 0.1% (about 500 to 1,000 mg/L) that is comprised of impurities originating from industries or from human water consumption. Wastewater treatment, therefore, involves removing only these impurities. If we see a product sold in the market—such as a detergent or a can of peaches—which boasts of even 95% purity, we will not think twice before buying it. Why then should we not reuse and recycle water? We must.

Did you also know that water is needed in almost everything we use, we do, and we eat? Table 1 on page 12 shows a summary of how much water we use every day on food and products we use in our daily lives. We do not even think about how much water is needed to make these products.

Did you also know that the human body contains 70% to 75% of water by weight? So a person weighing 160 lb contains about 115 lb of water. This is simply mind-boggling. To put it mildly, we cannot live without water. Water is life.

And there is another reason why we should reuse and recycle water. The population of our planet is projected to increase to more than 9 billion from the current 7.5 billion by the year 2040. In addition, standards of living continually are increasing all over the globe, which are further stressing our water resources. However, water quantities available for use are constant and always have been since the dawn of time. Water recycling is a resource that must be tapped to meet the continually growing water demand.

Stages of Treatment

There are five stages of wastewater treatment, some or all of which are necessary before water can be reused and recycled. How many stages should be employed before reuse depends upon the type of intended reuse. The purpose and level of treatment in all cases is the protection of public health. 

For example, if the water is going to be reused for growing alfalfa intended for animal feed, the requirements will be less stringent than if it is to be used to grow edible crops such as tomatoes or lettuce. Similarly, if the intended use is golf course irrigation at nighttime, the requirements would be less stringent than if the intended use is irrigation of school playgrounds used by children. Reuse for drinking purposes, whether direct or indirect potable applications, require the highest quality of recycled water, and all five stages of treatment—plus aggressive monitoring, redundancy and reliability in design and operation—would be required. Reuse applications between these two extremes, of which there are many, also will require fewer stages of treatment.

These five stages of wastewater treatment are:

  1. Preliminary treatment;
  2. Primary treatment;
  3. Secondary treatment;
  4. Tertiary treatment; and 
  5. Advanced treatment (AWT).

Different stages remove different constituents from wastewater. Preliminary treatment primarily removes grit and screenings; primary treatment removes organic and inorganic suspended matter; secondary treatment removes both suspended and dissolved organic substances, mostly by biological means; tertiary treatment, for the most part, removes particles not removed in previous stages of treatment; and advanced treatment removes refractory and nonbiodegradable materials. Disinfection with high doses of disinfectants or oxidants, such as ultraviolet light, ozone and/or hydrogen peroxide, prepares the water for many unrestricted reuse applications, including indirect potable reuse (IPR) and direct potable reuse (DPR).

Most of us are quite familiar with the first four stages of treatment because almost all wastewater treatment plants today employ them. However, the AWT stage is not that common. In water-scarce parts of the country, California as an example, AWT has been provided by a few large water agencies and is being considered by many others. This opens up other avenues for water reuse, such as IPR and DPR via groundwater recharge, storage in underground water aquifers, storage in large raw water reservoirs used as a source of drinking water supplies, and many others. AWT essentially removes refractory materials not removed in the first four stages of treatment, which are present at parts per trillion levels (ppt) in our wastewater. Industries are the sources that contribute to these refractory and recalcitrant materials (most often their products have proprietary formulations and contain chemicals that do not have to be mentioned and therefore are not disclosed); personal care products used in homes, such as fragrances and others; medicines and drugs; and antibiotics, pesticides, and various pharmaceutical compounds to name a few. These materials are believed to be carcinogens and endocrine disruptors, and they must be removed for IPR and DPR applications per regulations in many states and local jurisdictions to protect public health. 

If they are not removed, they could interfere with a body’s endocrine system and produce adverse developmental, reproductive, neurological, and immune effects in humans and wildlife over time.

To put in perspective what ppm, ppb and ppt mean, the following may be of interest: 

  • 1 part per million (ppm) approximately equals one drop of a chemical in 13 gal of water.
  • 1 part per billion (ppb) approximately equals one drop of a chemical in 13,000 gal of water (one residential swimming pool).
  • 1 ppt approximately equals one drop of a chemical in 13 million gal of water (1,000 residential swimming pools).

With new analytical methods available, these chemicals, even though present in ppb and ppt concentrations, can easily be measured. Their long-term effects on human health are not always fully known, but are of concern in water reuse for IPR and DPR.

Each stage of treatment produces a waste stream (screenings, grit, biosolids, sludge, etc.) that must be hauled away or treated to produce beneficial materials such as soil conditioners, compost and some form of aggregate with potential reuse applications in roads, land reclamation and others. Once again, there are state and federal requirements and standards that govern these uses with the sole objective to protect public health. In addition, anaerobic digestion of biosolids can produce up to
60 kW of electricity per 1 million gal per day of wastewater flow. New processes are being developed to increase this so that the wastewater treatment plants are close to being energy neutral.

Reuse Applications

State and local jurisdictions may have different requirements, and they must be determined before a reuse project is planned and implemented.

The method of applying water for irrigation-spray versus subsurface drip, for example, can make a difference in the treatment level  required. The public exposure to spray irrigation systems is much higher than subsurface drip irrigation, and thus may have higher levels of treatment required and associated application restrictions.

Future of Wastewater Treatment & Reuse

The future of wastewater treatment and reuse seems to be limitless. In general, advancements will occur in the following areas:

  • Direct and indirect potable use.
  • New technologies for nitrogen and phosphorus removal, which are of concern in reuse for lake replenishment and reservoirs, as well as discharge into sensitive water bodies.
  • Optimization and development of new membrane processes that can replace secondary clarifiers and tertiary filters to reduce capital cost and energy consumption.
  • New and cost-effective processes for removal of refractory and recalcitrant compounds, including studies to determine their impact on human health and wildlife.
  • New technologies for treating residuals and biosolids resulting from treatment of wastewaters with the aim of nutrient recovery (nitrogen and phosphorus), enhanced energy recovery from digestion, and near-complete recycling and reuse of biosolids.
  • Reducing energy and the carbon footprint of existing and new technologies to enhance sustainability and reduce the formation of greenhouse gases and the impact of global climate change.
  • Decentralized treatment facilities for optimal water reuse and recycling.
  • Public education to enhance acceptance of recycled water for various applications, including potable reuse.
  • Political will for resources on studying the above issues and the implementation of reuse projects.

Future wastewater treatment plants will be resource recovery plants not just in euphemism, but in actuality.

It is a tall order, no doubt. But as water sector engineers and planners have demonstrated repeatedly over the years, this is a task that is important and achievable. We have come a long way from septic tanks to modern-day plants employing complex technologies, so moving forward is certainly easier than the journey of the past.

About the Author

Madan Arora

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