Introduction

The biological treatment systems discussed to this point (ponds, trickling filters, and rotating biological contactors) have been around for years. The trickling filter, for example, has been used successfully since the late 1800s. The problem with ponds, trickling filters, and rotating biological contactors is that they are temperature sensitive and remove less biochemical oxygen demand (BOD); also, trickling filters, for example, cost more to build than the activated sludge systems that were later developed.

The activated sludge process follows Key Point: Although trickling filters and other primary settling. The basic components systems cost more to build than activated sludge

„ ^ j i j . . . systems, it is important to point out that activated of an activated sludge sewage treatment

® _ ® sludge systems cost more to operate because of system include an aeration tank and a the need for energy to run pumps and blowers.

secondary basin, settling basin, or clarifier (see Figure 15.1). Primary effluent is mixed with settled solids recycled from the secondary clarifier and then introduced into the aeration tank. Compressed air is injected continuously into the mixture through porous diffusers located at the bottom of the tank, usually along one side. Wastewater is fed continuously into an aerated tank, where the microorganisms metabolize and biologically flocculate the organics. Microorganisms (activated sludge) are settled from the aerated mixed liquor under quiescent conditions in the final clarifier and are returned to the aeration tank. Left uncontrolled, the number of organisms would eventually become too great; therefore, some must periodically be removed (wasted). The concentrated solids at the bottom of the settling tank that must be removed from the process are known as waste activated sludge (WAS). Clear supernatant from the final settling tank is the plant effluent.

Figure 15.1 The activated sludge process.

15.1.1 Activated Sludge Terminology

To better understand the discussion of the activated sludge process presented in this chapter, you must understand the terms associated with the process. Some of these terms have been used and defined earlier in the text, but we include them here again to refresh your memory. Review these terms and remember them. They are used throughout the discussion.

Absorption—A body taking in or receiving a substance by molecular or chemical action and subsequent distribution throughout the absorber.

Activated—Refers to speeding up a reaction. When applied to sludge, it means that many aerobic bacteria and other microorganisms are in the sludge particles.

Activated sludge—A floc or solid formed by the microorganisms. It includes organisms, accumulated food materials, and waste products from the aerobic decomposition process.

Activated sludge process—A biological wastewater treatment process in which a mixture of influent and activated sludge is agitated and aerated. The activated sludge is subsequently separated from the treated mixed liquor by sedimentation and is returned to the process as needed. The treated wastewater overflows the weir of the settling tank in which separation from the sludge takes place.

Adsorption—The adherence of dissolved, colloidal, or finely divided solids to the surface of solid bodies when they are brought into contact.

Aeration—Mixing air and a liquid by one of the following methods: spraying the liquid in the air, diffusing air into the liquid, or agitating the liquid to promote surface adsorption of air.

Aerobic—Refers to a condition in which free, or dissolved, oxygen is present in the aquatic environment. Aerobic organisms must be in the presence of dissolved oxygen to be active.

Bacteria—Single-cell plants that play a vital role in stabilization of organic waste.

Biochemical oxygen demand (BOD)—A measure of the amount of food available to microorganisms in a particular waste. It is measured by determining the amount of dissolved oxygen used up during a specific time period (usually 5 days, expressed as BOD5).

Biodegradable—From "degrade" (to wear away or break down chemically) and "bio" (by living organisms); put it all together, and you have a substance, usually organic, that can be decomposed by biological action.

Bulking—A problem in activated sludge plants that results in poor settleability of sludge particles.

Coning—A condition that may be established in a sludge hopper during sludge withdrawal, when part of the sludge moves toward the outlet while the remainder tends to stay in place; development of a cone or channel of moving liquids surrounded by relatively stationary sludge.

Decomposition—Generally, in waste treatment, refers to the changing of waste matter into simpler, more stable forms that will not harm the receiving stream.

Diffused air aeration—Occurs when an air activated sludge plant compresses air and discharges the air below the water surface to the aerator through some type of air diffusion device.

Diffuser—A porous plate or tube through which air is forced and divided into tiny bubbles for distribution in liquids; commonly made of carborundum, aluminum, or silica sand.

Dissolved oxygen—Atmospheric oxygen dissolved in water or waste-water; usually abbreviated as DO.

Note: The typical required DO for a well-operated activated sludge plant is between 2.0 and 2.5 mg/L.

Facultative—Facultative bacteria can use either molecular (dissolved) oxygen or oxygen obtained from food materials; in other words, facultative bacteria can live under aerobic or anaerobic conditions.

Filamentous bacteria—Organisms that grow in thread or filamentous form.

Food-to-microorganisms (F/M) ratio —A process control calculation used to evaluate the amount of food (BOD or COD) available per pound of mixed liquor volatile suspended solids. This may be written as:

Food _ BOD (lb/day) _ Flow (MGD) x BOD (mg/L) x 8.34 lb/gal Microorganisms _ MLVSS (lb) _ Volume (MG) x MLVSS (mg/L) x 8.34 lb/gal

Fungi—Multicellular aerobic organisms.

Gould sludge age—A process control calculation used to evaluate the amount of influent suspended solids available per pound of mixed liquor suspended solids.

Mean cell residence time (MCRT)—The average length of time a mixed liquor suspended solids particle remains in the activated sludge process; may also be referred to as sludge retention time (SRT).

MCRT (da s) - Solids in Activated Sludge Process (lb) Solids Removed from Process (lb/day)

Mixed liquor—The contribution of return activated sludge and waste-water (either influent or primary effluent) that flows into the aeration tank.

Mixed liquor suspended solids (MLSS)—The suspended solids concentration of the mixed liquor. Many references use this concentration to represent the amount of organisms in the activated sludge process.

Mixed liquor volatile suspended solids (MLVSS)—The organic matter in the mixed liquor suspended solids. This can also be used to represent the amount of organisms in the process.

Nematodes—Microscopic worms that may appear in biological waste treatment systems.

Nutrients—Substances required to support plant organisms. Major nutrients are carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus.

Protozoa— Single-cell animals that are easily observed under the microscope at a magnification of 100x. Bacteria and algae are prime sources of food for advanced forms of protozoa.

Return activated sludge (RAS)—The solids returned from the settling tank to the head of the aeration tank.

Rising sludge—Occurs in the secondary clarifiers or activated sludge plant when the sludge settles to the bottom of the clarifier, is compacted, and then rises to the surface in a relatively short time.

Rotifers—Multicellular animals with flexible bodies and cilia near their mouths used to attract food. Bacteria and algae are their major source of food.

Secondary treatment—A wastewater treatment process used to convert dissolved or suspended materials into a form that can be removed.

Settleability—A process control test used to evaluate the settling characteristics of the activated sludge. Readings taken at 30 to 60 minutes are used to calculate the settled sludge volume (SSV) and the sludge volume index (SVI).

Settled sludge volume—The volume (mL/L or percent) occupied by an activated sludge sample after 30 or 60 minutes of settling; normally written as SSV with a subscript to indicate the time of the reading used for calculation (SSV30 or SSV60).

Shock load—The arrival at a plant of a sufficient quantity or strength of a waste toxic to organisms to cause operating problems, such as odor or sloughing off of the growth of slime on the trickling filter media. Organic overloads also can cause a shock load.

Sludge volume index—A process control calculation used to evaluate the settling quality of the activated sludge. Requires the SSV30 and mixed liquor suspended solids test results to calculate:

Solids—Material in the solid state:

Dissolved— Solids present in solution; solids that will pass through a glass fiber filter.

Fixed—Also known as the inorganic solids; the solids that are left after a sample is ignited at 550°C for 15 minutes.

Floatable—Solids that will float to the surface of still water, sewage, or other liquid; usually composed of grease particles, oils, light plastic material, etc. Also called scum.

Nonsettleable—Finely divided suspended solids that will not sink to the bottom in still water, sewage, or other liquid in a reasonable period, usually 2 hours. Nonsettleable solids are also known as colloidal solids.

Suspended— Solids that will not pass through a glass fiber filter.

Total—Solids in water, sewage, or other liquids; includes suspended solids and dissolved solids.

Volatile— Organic solids; measured as the solids that are lost on ignition of the dry solids at 550°C.

Waste activated sludge (WAS)—The solids being removed from the activated sludge process.

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