A water conditioning professional works diligently to provide a water supply that is aesthetically pleasing, safe from chemical and biological hazards, free of damaging constituents and mineral salts, and pleasant to the olfactory senses. The water is treated with careful consideration to satisfy the needs of the water-using client. Then, some of the clients contaminate this elegantly engineered water with chemicals, soaps, food waste and other unpleasantness — just to flush it down the drain without a second thought. With the Earth’s staggering human population explosion comes the need to reuse the finite supply of fresh water available to sustain life. Present and future water professionals must understand the downstream implications and processes related to wastewater to stay current with emerging technologies for recovery and reuse. This article will discuss some of the very first stages of wastewater management as it applies to most public sewage water systems.

Types of used water

To discuss wastewater, one should understand the two primary classifications of used water. Water going to the drain falls under the definitions for “greywater” and “blackwater.” A very simple definition of greywater is: Wastewater from bathing, washing clothes and bathroom sinks.1 Greywater requires fewer steps for reuse in applications such as irrigation and flushing toilets. It has much lower levels of challenging biological material than blackwater. Greywater treatment can be on-site or centralized, but it is separate from water going to a managed sewage treatment plant.

Blackwater by definition is: All types of wastewater from residential, commercial and industrial sites, including human waste, kitchen sinks and dishwashers, greywater, combined sewer and stormwater from rain runoff, and other water containing biological waste matter. Blackwater is also referred to as sewage, wastewater and sewer water.2

There are three levels of treatment with blackwater systems: Primary, secondary (biological) and tertiary treatment. Each level takes the water to a cleaner stage in preparing it for reuse.

Tertiary treatment is an additional treatment following secondary treatment. Tertiary treatments can remove more than 99 percent of all the impurities from blackwater, producing an effluent quality approaching drinking water standards. Tertiary technologies are varied and usually very expensive. Systems such as membrane bioreactors (MBR) and ultrafiltration (UF) require a high level of technical aptitude and special training to operate. Tertiary treatment is a complex topic requiring great detail and is worthy of more discussion in another article.

Primary treatment

At this stage, blackwater (sewage) leaves its point of origin and flows to central lift stations or simply flows to the treatment facility where it first passes through bar screen to remove the suspended, floating and gross solids (including household items flushed down the toilet to clothing, drink bottles, dead animals, logs, etc., entering the system from uncovered/unprotected access). Much of this material will not further biodegrade and is simply hauled away for disposal. The sewage then moves to a holding area for several hours to allow grit to settle out for removal and liquid sewage to pass over to the primary clarifier. In the primary clarifier, sewage sludge will settle to the bottom of the the collection basin.

These solids can go directly to dewatering and disposal or go to a “sludge digester” where bacteria further break down the sludge before it is dewatered for disposal or reuse. The liquids from the clarifier flow out of the top of the clarifier basin and transfer to secondary treatment.

In the primary treatment stage, the goal is to produce a liquid waste with reduced biochemical oxygen demand (BOD) levels for the secondary treatment. BOD is the amount of dissolved oxygen required by microorganisms, e.g., aerobic bacteria, to break down the organic material present in a wastewater supply. Waste management authorities measure BOD in a given water sample at a specific time period and at a certain temperature. There is a BOD5, where the sample sets for five days in the presence of microorganisms to determine the oxygen demand, and a BOD7, which is a seven-day test. A clear lake might have a BOD of 2 ml/l, where raw sewage may be in the hundreds and wastewater from a food processing plant can be in the thousands. Managed wastewater districts use BOD to set fees and surcharges for receiving sewage from various sources. Industrial and processing plants often receive surcharges from their wastewater disposal district for high BOD discharge. A wastewater’s BOD, in essence, is an indication of that water’s level of pollution.

BOD level (ppm)

Water quality

1 to 2 Very good
There will be very little organic waste in this water.
3 to 5 Fair
Moderately clean water.
6 to 9 Poor: Somewhat polluted
Indicates organic matter present with bacteria decomposition.
100 or greater Very poor: Extremely polluted
Contain significant organic waste.

The problem of too much BOD is the treatment plant may struggle in getting the water clean before it is discharged. Remember, a treatment plant will have a finite capacity. If that capacity is breached before the process completes, untreated waste can discharge to secondary treatment. If the entire system is overrun with excess flow, high BOD water (overflow) can bypass and discharge — usually to a surface water source — and bad things happen as a result. A sudden release of high BOD waste can cause fish kills because the water is robbed of its oxygen. Heavy organic pollution has the potential to create “dead zones” where fish and other aquatic live cannot survive. The 2014 annual spring-summer dead zone in the Gulf of Mexico, at the tail waters of the Mississippi, encompassed 5,052 square miles — or the size of Connecticut. This “low-oxygen water” forms from agriculture fertilizers and wastewater entering the Gulf from the Mississippi River.3 Partially treated wastewater, high in organic matter, carries nitrates and phosphate which act as nutrients for aquatic plants. Algae and other undesirable aquatic plants grow and thrive in these conditions and add to the overall organic load on the water system. These algae blooms suck up the oxygen creating these conditions.

Some sewage treatment plants discharge water into natural water systems that provide drinking water for communities downstream of the discharge point. The U.S. Environmental Protection Agency’s (EPA) Clean Water Act puts regulations on the TMDLs (total maximum daily loads) going into natural waterways, but problems can happen. In August of 2013, 3.5 million gallons of untreated wastewater discharged into the Mississippi River due to a power outage in North St. Louis.4 In Memphis, Tennessee, their wastewater plant discharges 82 million gallons of treated wastewater into the Mississippi River daily. There is phosphate foaming on the river surface downstream of the plant discharge. The city is working on a project to make the Mississippi River’s water along the Memphis riverfront area cleaner and safer, but currently there are issues.

Primary treatment can reduce BOD levels 20 to 30 percent and suspended solids levels 50 to 60 percent. The less BOD going into secondary treatment, the better the chances are of having very low levels of pollutants discharging from the plant or to tertiary treatment.5

Secondary treatment

Secondary treatment digests organic matter that flows from primary treatment. Air is added to the wastewater to increase the oxygen levels. They do this to provide a healthy environment for the microbes (referred to as “bugs” by the wastewater industry) to thrive. The bugs consume the organic matter as food, converting it to carbon dioxide, water and energy for their own growth and reproduction. A certified operator must balance the bugs with their food source and their oxygen needs. Too little organic matter and too many bugs means the bugs starve and poor quality water leaves the system. Too much food with too few bugs also results in poor quality effluent leaving secondary treatment due to elevated levels of organics remaining in the water.6 These bugs are aerobic, meaning they need oxygen to breathe. With little oxygen the bugs die, again resulting in poor water quality.

The certified operator manages their recipe of organics, bugs and oxygen in a secondary clarifier. The bugs work until there is no oxygen remaining. At that point, they stop working and settle to the bottom of the clarifier where they are returned to the head of the aeration tank to start working on a fresh supply of incoming wastewater. Secondary treatment removes about 85 percent of the suspended solids and BOD.

The bugs (bacteria) used in this secondary treatment can be autotrophic or heterotrophic. An aerobic autotroph can synthesize its own food from inorganic substances. The water coming into the secondary treatment, for example, has high levels of ammonia (NH3) in it from urine. They use carbon dioxide (CO2) as a source of carbon and ammonia as a source of hydrogen to reduce carbon. The result is the elimination of ammonia and the creation of nitrite (NO2) and nitrate (NO3). This is called “nitrification.” To “denitrify” the water (to remove the nitrate) the operator uses heterotrophic bacteria. Heterotrophs cannot product their own food. They use organic carbon (dead organic matter) for food. In an anoxic (low oxygen) environment, this anaerobic bacteria get its oxygen from the NO3 producing harmless nitrogen gas (N2O) which off gases to atmosphere. Removing nitrate from the wastewater removes a needed nutrient for algae growth.

The other nutrient in wastewater for algae is phosphorus. Adding chemicals such as lime will cause the phosphates to precipitate and settle out of the wastewater. This precipitate is then removed in either the primary or secondary settling basin.7

In some plants, the water leaves secondary treatment for media filtration and disinfection. In other cases, it moves on to tertiary treatment and possible reuse. Wastewater treatment plants vary in their exact handling of wastewater based on the severity of the waste and the desired end product. Some involve additional steps and components, while others are extremely simple in their approach. A septic tank with a drain field is an example of a simple approach. A blackwater to drinking water plant involves numerous technologies and high levels of management. It is important to remember microbial components are part of any waste process at some point.

This is only a generic look at managing blackwater. Depending on the source and level of contamination, it quickly becomes more complicated and more expensive. It is not the job of a POE/POU treatment professional to be an expert in wastewater treatment, but it is advisable to learn more about the downstream processes to remain current on the challenges and changes affecting the water treatment industry.

Sources

  1. http://greywater.sustainablesources.com/; Jan. 23, 2015.
  2. https://whollyh2o.org/blackwater/item/381-blackwater-defined.html; Jan. 23, 2015.
  3. Aug. 5, 2014; http://www.cnn.com/2014/08/05/tech/gulf-of-mexico-dead-zone/; retrieved Feb. 2, 2015.
  4. Jan. 19, 2014; http://www.commercialappeal.com/news/local-news/memphis-takes-aim-at-mississippi-river-pollution; retrieved Feb. 1, 2015.
  5. http://water.worldbank.org/shw-resource-guide/infrastructure/menu-technical-options/wastewater-treatment; Jan. 23, 2015.
  6. https://www.mymanatee.org/home/government/departments/utilities/wastewater-system/ww-treatment-plants.html; retrieved Feb. 2, 2015.
  7. August 2006; http://www.water.rutgers.edu/Projects/trading/p-trt-lit-rev-2a.pdf; retrieved Feb. 2, 2015.

Matthew Wirth is the general manager of Pargreen Water Technologies (an industrial system and service group based in Chicago) and a water professional with over 34 years of experience. He holds a Bachelor of Arts in communication and organizations management from Concordia University in St. Paul, Minnesota, and received his engineering training at the South Dakota School of Mines and Technology in Rapid City, South Dakota. Wirth is an experienced trainer and writer with expertise in industrial water applications, sales and water treatment system distribution. He can be reached by email at mwirth@pargreen.com, www.pargreenwater.com or by phone at 630-433-7766.