Objective

In this lesson we will learn the following:

• What are the different sources of wastewater.
• What are the different steps included in the treatment process.

Reading Assignment

Along with the online lesson, read Chapter 1: The Treatment Plant Operator, Chapter 2: Why Treat Wastes? and Chapter 3: Wastewater Treatment Facilities, in your textbook Operation of Wastewater Treatment Plants Volume I .




Lecture

Introduction

This lesson begins the second half of ENV 110.  This half of the course is concerned with the treatment of wastewater. Before beginning this lesson, you should have taken the midterm exam and visited a water treatment plant to conclude the water treatment portion of the course.

Lessons 1 through 11 considered the water treatment process. Water flows from the source through the treatment plant and to the consumer.  But what happens to the water after it reaches the consumer?

The second half of this course will concern itself with wastewater - the used water and solids from a community as well as the storm water which runs off streets and other surfaces during storms.  

You will remember that water is naturally cleaned and reused as part of the hydrologic cycle in the outside world.  In the human world, water is also cleaned and reused.  Our wastewater is channeled to a wastewater treatment plant where it is cleaned and released back into lakes and rivers. This water reenters the hydrologic cycle and will eventually be pumped back up by another water treatment plant to be purified and released to customers.  

 

 

Introduction to Wastewater

Wastewater or sewage is the byproduct of many uses of water. There are the household uses such as showering, dishwashing, laundry and, of course, flushing the toilet. Additionally, companies use water for many purposes including processes, products, and cleaning or rinsing of parts. After the water has been used, it enters the wastewater stream, and it flows to the wastewater treatment plant.

You can classify wastewater as domestic, industrial, or storm, according to its origin. Domestic sources include water used for normal activity in homes, businesses and institutions. Domestic wastewater is readily treatable. The character of industrical wastewater depends on the type of industry using the water. Some industrial wastewaters can be treated the same as domestic wastes without difficulty. Others may contain toxic substances or high percentages of organic materials or solids which make treatment difficult. In such cases, the industrial plant may have to pretreat its wastewater to remove these pollutants or reduce them to treatable levels before they are accepted into a publicly-owned treatment facility. Storm water often goes to a treatment plant, although it is usually low in pollutants. Great amounts of storm water can interfere with treatment efficiency in two ways: Storm water may cause too much dilution of the wastewater. At the same time, it may cause hydraulic overloading of the plant. In most cases, wastewater systems now call for separate storm sewers.

We need to remove the wastewater pollutants to protect the environment and protect public health. When water is used by our society, the water becomes contaminated with pollutants. If left untreated, these pollutants would negatively affect our water environment. For example, organic matter can cause oxygen depletion in lakes, rivers, and streams. This biological decomposition of organics could result in fish kills and/or foul odors. Waterborne diseases are also eliminated through proper wastewater treatment. Additionally, there are many pollutants that could exhibit toxic effects on aquatic life and the public.

The sewer or collection system is designed so that it flows to a centralized treatment location. The collection system is compromised of smaller sewers with a diameter of about four inches. As more homes and companies are connected along the system, the pipes become larger in diameter. Where gravity systems are not practical, pumping stations are often included to lift the wastewater. Many systems experience problems during wet weather periods with inflow and infiltration. Wet weather operating periods typically occur when the snow melts in the spring and/or during heavy rainstorms. Water resulting from snowmelt or storms shoudl flow into a storm water system and not into the sanitary sewer system. Unfortunately, this isn't always the case.

The wastewater continues to flow through the collection system and eventually reaches the wastewater treatment plant. Upon reaching the plant, the flow first encounters preliminary treatment . Preliminary treatment is followed by primary treatment, then secondary treatment, and perhaps advanced or tertiary treatment. The solids or "sludge" removed from the wastewater stream also needs to be treated.

 

 

In the Treatment Plant

In the treatment plant there are many steps involved in treating wastewater. Below is a quick overview of the possible steps involved. We will learn more about each step as the course goes on.

 

The general principle in wastewater treatment is to remove pollutants from the water by getting them either to settle or to float, and then removing this material. Some pollutants are easily removable. Others must be converted to a settleable form before they can be removed. Treatment facilities are designed in stages. Each stage either removes particles from the wastewater or changes dissolved and suspended material to a form that can be removed. A modern wastewater treatment plant may include these stages:

• influent
• preliminary treatment
• secondary treatment
• tertiary treatment
• disinfection and effluent discharge

 

 

Influent

Inflow is water from a sump pump or a roof leader. This is relatively clean water that should be discharged to a storm water system. In many communities, there are "combined sewers" that carry street runoff, as well as wastewater. Infiltration is water from high groundwater levels. Older sewer pipes may have leaking joints or cracks that allows the water to enter the system. Infiltration usually occurs in the spring when melting snow and rain saturate the ground.

 

 

Preliminary Treatment

Preliminary treatment processes are the first processes that the wastewater encounters. This typically involves flow measurements so that the operator can quantify how much wastewater is being treated. Flow monitoring is commonly followed by screenings removal. Screenings are string like materials and large foreign objects like sticks or perhaps an errant golf ball. These materials need to be removed because they can damage machinery or clog processes. Screenings can be removed using bar screens and other devices designed for this purpose.

The next process in preliminary treatment is grit removal. Grit is comprised of inorganic material such as sand, gravel, eggshells, etc. It is desirable to remove grit to prevent wear and abrasion on pumps and other mechanical equipment. Grit can also plug lines and pipes. In this influent area, sampling equipment is often used to collect small portions of the wastewater for analysis. Sampling enables the operator to determine the pollutant loadings entering the plant (influent). Preliminary treatment commonly includes raw sewage pumps. Screening and grit removal are important to the proper operation of the raw sewage pumps. These materials will cause clogging and cause wear on the internal parts. These raw sewage pumps deliver the flow to the next phase of treatment: Primary Treatment.

 

 

Primary Treatment

Primary treatment is a physical settling process that removes solids. Wastewater that enters the primary settling tank (or clarifier) is slowed down to enable the heavier solids to settle to the bottom. Ligher materials, such as grease, will float to the top of the tank. Settling tanks are designed with mechanisms to remove both the settled solids, as well as the floating solids. Primary clarifiers are either circular or rectangular. Both types work equally well when properly designed and maintained. Not all plants have primary treatment.

Primary treatment generates primary sludge. The sludge is removed and pumped to the solids treatment process for ultimate removal. What's left after we remove the pollutants that settle and float? The wastewater still has solids remaining after primary treatment. These solids are either dissolved or suspended. Dissolved solids are very small solids. You cannot see the solids but they are there. Suspended solids can be likened to the same ends of a magnet. The solids repel each other. These solids are small, but are visible to the human eye. We remove these dissolved and suspended solids through the next phase of treatment: Secondary Treatment.

 

 

Secondary Treatment

Secondary treatment is a biological treatment process used to stabilize the dissolved solids. Microorganisms, such as bacteria, feed on the organic solids (food) in the wastewater and convert the organics into a cellular or biological mass that can later be removed. These biological processes are aerobic processes. Oxygen must be provided for these aerobic organisms to work properly and efficiently. An integral part of secondary treatment processes is another set of settling tanks or clarifiers. These secondary clarifiers (final clarifiers) remove the biological treatment.

There are many different kinds of secondary processes that can be employed. A very common secondary process is known as activated sludge. In activated sludge treatment, the wastewater is mixed with organisms that are returned from the secondary clarifiers. There is a continuous return of organisms from the secondary clarifiers. This is called return sludge or return activated sludge. Oxygen is provided in the aeration tank either by blowers and diffusers or by a mechanical mixing process. A variation of the activated sludge process that is becoming more popular is known as Sequential Batch Reactors (SBRs). This process differs from the more conventional activated sludge systems in that it also uses the aeration tank as a settling tank. This is accomplished by turning off the air to the diffusers or the mixers and allowing the solids to separate from the wastewater. During this settling period, the flow is diverted into a second SBR tank for continuous treatment. Advantages of this SBR process include a relatively small footprint and the capability of removing nutrients (both nitrogen and phosphorus)

Lagoon systems are also a form of biological or secondary treatment. These lagoons systems are used where there is a lot of land available and/or the wastewater flows (quantities) are low. Lagoons are constructed with lined earthen bottoms and are less expensive to construct than are activated sludge processes that use concrete tanks. Limitations of lagoons may include excessive algae growth (solids violations) and poor performance in the winter.

Another type of secondary treatment is known as fixed film processes. Fixed film processes consist of two types. Trickling filters or Rotating Biological Contactors (RBC). Trickling filters are sometimes called Bio Towers. Trickling filters are beds with a synthetic material (media). An under-drain system and a rotary distribution system apply the wastewater to the media. The microorganisms grow attached to the rocks or synthetic media as opposed to liquid suspension in the activated sludge. A circular rotary distributor moves over the media bed and the wastewater flows over the media, it comes into contact with the microorganisms and picks up oxygen. When the biological growth becomes too thick, it falls off the media and flows with the wastewater to a secondary settling tank for removal. Many trickling filter plants that originally were designed with rock media have changed to the more efficient plastic media.

The RBC is similar to the trickling filter in that it uses an attached biological growth. An RBC has panels that are circular and mounted to a shaft. The wastewater flows into a basin beneath the media and the media rotates with the shaft. The microorganisms are contacted with the wastewater. Since the RBCs expose the media to the air, oxygen is picked up and transferred into the growth. RBCs have low energy requirements. These systems need to be protected from cold weather by a building. Intermittent sand filters are employed in some smaller applications. As wastewater passes through the filter bed, solids are removed. Microorganisms grow in the removed solids layer and provide biological treatment of the wastewater as it flows through the sand bed. The sand will need to be replaced at some point in time. Additionally, these sand filter systems generally perform poorly in the winter. All of the secondary treatment processes produce biosolids. These biosolids are pumped to the solids treatment system for further processing.

 

 

Disinfection

In many plants, the next process is called disinfection. Disinfection means the inactivation of disease-causing organisms. It is sometimes confused with sterilization which means the killing of all organisms. In disinfection, the wastewater following secondary treatment is usually treated in one of two ways: (1) chlorination or (2) ultraviolet radiation.

Chlorination involves the use of chlorine, either in the form of a gas (less common today), or as a liquid (sodium hypochlorite). The chlorine oxidizes the microorganisms. The effectiveness of this process is monitored by testing the fecal coliform group. This indicator group of microorganisms are easy to grow in a laboratory and are tougher to kill than pathogens. Some chlorination systems also have dechlorination systems to remove any residual chlorine.

Ultraviolet (UV) disinfection systems contact the treated secondary wastewater with UV light bulbs that are encased in clear housings. The UV light kills pathogenic organisms by using a germicidal photochemical wavelength. Unlike chlorination, UV leaves no residual in the wastewater with which to be concerned. Plants that use UV must either have dual UV systems or have chlorination as a backup. Additionally, these UV systems are energy consumptive.

 

 

Advanced Treatment

Some treatment plants may be required to remove nutrients (nitrogen and phosphorus) due to the possible negative impacts on the receiving stream (e.g., ammonia toxicity to fish). Advanced treatment processes are used to remove nutrients, additional solids, and/or biochemical oxygen demand. Advance treatment provides very high level of treatment that goes beyond secondary treatment. In the case of nitrogen removal, the processes are biological. For phosphorus removal, chemical additives are normally required.

 

 

Solids Handling

Solids that settle out in the primary and secondary clarifiers are referred to as sludge. Sludge from biological treatment processes (e.g., activated sludge) are referred to as biosolids. Sludge is the byproduct of treating the liquis wastewater. Proper solids handling is of paramount importance. If sludge is not removed, problems will occur in other areas of the plant. Excess solids can also lead to State Pollution Discharge Elimination System Permit violations and odor problems. There are many different options available for solids handling. Local conditions usually dictate which option is best for your particular facility. General categories of sludge handling include digestion processes, hauling of liquid sludge to a larger treatment plant, thickening, dewatering by mechanical means (belt filter presses, centrifuges), incineration, land filling, and land application.

The treated wastewater is referred to as effluent. The effluent is discharged to a water body such as a lake, river, stream, or groundwater. Conditions contained in the permit are designed to minimize the impact that the effluent may have on the receiving stream.

 

 

Contaminants Found in Untreated Wastewater

Fresh domestic untreated or raw wastewater has a musty odor, a pH range of 6.5 to 8.0 and is grayish-brown in color. Contaminants typically found in untreated wastewater can be broadly lumped into four basic classes:

• Organic Contaminants
• Inorganic Contaminants
• Pathogens
• Other Contaminants

 

 

Organic Contaminants

Organic contaminants are derived from animals and plants, or may be manufactured chemical compounds. However, all organics contain carbon. Organic contaminants can be biodegradable, which means that the contaminants can be consumed by bacteria and other microorganisms. In the process of being consumed, these organics will exert an oxygen demand which can be measured as the Biochemical Oxygen Demand (BOD) of the wastewater. Some organic contaminants, known as refractory organics, are resistant to biodegradation. In a typical domestic wastewater treatment plant, the BOD of the influent raw wastewater ranges around 200 to 250 mg/L. Sudden or drastic increases in BOD loadings at a wastewater plant are examples of organic shock loadings and may lead to treatment plant upsets. When shock loads are suspected, plant representatives should begin looking for potential sources of elevated organic loads.

 

 

Inorganic Contaminants

Inorganic contaminants are not biodegradable, but may be nutrients necessary for microorganisms to live. These are typically chemical compounds (priority pollutants) or metals that are either present in the wastewater as suspended solids or as dissolved inorganics. Examples of inorganic contaminants include:

• The sodium chloride byproduct from the water softening process adds to the total dissolved solid content in water.
• Nutrients such as phosphorus and ammonia-nitrogen. Both of these nutrients are typically found in domestic sewage, internal recycle flows (belt press filtrate or anaerobic digester supernate), and trucked in wastes.
• The filter backwash from drinking water facilities is often high in suspended solids and low in organic loading.
• Street cleaning or sidewalk washing introduces soil, sand, or grit.
• Copper (a heavy metal) dissolved from household plumbing.
• Other toxic metals from industrial processes.

 

 

Pathogens

Pathogens are disease-causing organisms including bacteria and viruses that can be deposited in the wastewater through human or animal wastes, or from improperly handled hospitals wastes. Proper hygiene is extremely important when working around wastewater. Because the potential disease is so great, it is important that wastewater be treated and disinfected to inactivate the pathogens prior to discharge to the receiving stream. It is particularly important if the receiving stream is used for recreational purposes such as boating, swimming and fishing, or as a drinking water source. Examples of diseases caused by pathogens that may be found in wastewater include:

• Typhoid
• Cholera
• Dysentery
• Polio
• Hepatitis

 

 

Types of Solids

Below is a typical solids concentration in raw wastewater.

 

 

Total Solids

• Total dissolved and suspended organic and inorganic residue left after evaporation, expressed in mg/L.
• Total solids include both dissolved and suspended materials.
• Suspended solids include both nonsettleable, or colloidal solids, and settleable materials.
• Total solids concentration in the figure above is 720 mg/L.

 

 

Dissolved Solids

• Solids which will pass through a standard glass fiber filter.
• Dissolved solids weight is the difference in weight between total solids and suspended solids.
• When a sample is filtered through fine mesh filter the suspended solids are captured on the filter pad and the dissolved solids will remain in the water passing through the filter.
• To determine the weight of dissolved solids, sample the water that passed through the filter. Evaporate the sample and weigh residue to determine weight of dissolved solids.
• Dissolved solids concentration from the figure above is 500 mg/L.

 

 

Suspended Solids

• Suspended solids are the solids that are captured on the filter pad, or the difference between the total and dissolved solids content of the sample.
• Includes solids which will settle or float in a clarifier and the lighter nonsettleable (colloidal) solids.
• The type of suspended solids is determined by size, shape, and weight.
• Suspended solids concentration in the figure above is 220 mg/L.

 

 

Settleable Solids

• A portion of the Suspended Solids.
• Large size particles settle more rapidly.
• Settleable solids are estimated before designing settling basins, sludge pumps and sludge handling facilities.
• Measuring settleable solids enables calculation of basin efficiency of removal of solids.
• Imhoff cone measures solids in mL/L.

 

 

Nonsettleable (Colloidal) Solids

• A portion of Suspended Solids.
• The colloidal solids will not settle but will remain in suspension after the settleable solids have precipitated out. The removal of colloidal solids usually requires the addition of a chemical flocculating agent or filtration.
• Calculated by subtracting the weight of settleable solids from the weight of suspended solids.
• Nonsettleable solids concentration in the figure above is 70 mg/L.

 

 

Floatable Solids

• Floatable solids are typically nonsettleable solids that make their way to the surface of a tank or stream.
• There is no standard for measuring and evaluating floatable solids.
• Typically made up of fat or grease particle and make up the scum. Scum is most easily removed by surface skimming equipment on the primary or secondary clarifiers.
• They are undesirable and unsightly, and can cause odors.

 

 

 

Effects of Wastewater Discharges

A discharge is the release of treated or untreated wastewater into a receiving stream. A discharge may occur from a treatment plant or from an overflow in the collection system. Untreated wastewater discharge can create several undesirable conditions. These include oxygen depletion and odor production in the stream, negative effects on human health, and sludge and scum accumulations.

 

 

Oxygen Depletion and Odor Production

The dissolved oxygen (DO) content of a stream will depend on the temperature and the flow characteristics. Cold water can retain higher dissolved oxygen content than warm water. As water temperatures increase, dissolved oxygen levels will decrease. Turbulent flow will add more dissolved oxygen to the stream than non-turbulent flow. The desired oxygen level to sustain living creatures, including aquatic life, is 5 mg/L. Organic waste is discharged to the receiving stream causing bacteria numbers to increase, as does oxygen use. When oxygen is used faster than it is replenished, aquatic life can die from insufficient oxygen. Anaerobic bacteria remove oxygen from sulfate. The sulfate is reduced to sulfide, which can combine with hydrogen in water to produce hydrogen sulfide, which produces a rotten egg odor.

 

 

Review

You can classify wastewater as domestic, industrial, or storm, according to its origin. In the treatment plant there are many steps involved in treating wastewater. The general principle in wastewater treatment is to remove pollutants from the water by getting them either to settle or to float, and then removing this material. A modern wastewater treatment plant may include these stages:

• influent
• preliminary treatment
• primary treatment
• secondary treatment
• disinfection and effluent discharge
• advanced treatment

 

 

Assignment

Please complete the Labster wastewater treatment plant tour simulation. You must be logged into Canvas to complete this assignment. Make sure you choose the appropriate semester.

 

 

Quiz

Answer the questions in the Lesson 12 quiz.   You will need to log into Canvas to take the quiz. You may take the quiz 3 times, if needed, and an average will be taken from your attempts for final grade calculation. Make sure you choose the appropriate semester.