Vocabulary Primary Treatment (Sedimentation/Flotation) Preliminary Treatment The removal of metal, rocks, rags, sand, eggshells, and similar materials which may hinder the operation of a treatment plant. Preliminary treatment is accomplished by using equipment such as racks, bar screens, comminutors, and grit removal systems. Coagulants Chemicals that cause very fine particles to clump (flock) together into larger particles. This makes it easier to separate the solids from the water by settling, skimming, draining, or filtering. Flocculation The gathering together of fine particles after coagulation to form larger particles by a process of gentle mixing. Colloids Very small, finely divided solids (particles that do not dissolve) that remain dispersed in a liquid for a long time due to their small size and electrical charge. When most of the particles in the water have a negative electrical charge, they tend to repel each other. This repulsion prevents the particles from clumping together, becoming heavier, and settling out. Clarifier Settling Tank, Sedimentation Basin. A tank or basin in which wastewater is held for a period of time during which the heavier solids settle to the bottom and the lighter material floats to the water surface. Detention Time The time required to fill a tank at a given flow or the theoretical time required for a given flow of wastewater to pass through a tank. Flights Scraper boards, made from redwood or other rot-resistant woods or plastic, used to collect and move settled sludge or floating scum. Hydraulic Loading Hydraulic loading refers to the flows (MGD or cu/ft/day) to a treatment plant or treatment process. Detention times, surface loadings, and weir overflow rates are directly influenced by flows. - 10 -
Launders Sedimentation tank effluent troughs, consisting of overflow weir plates. Short Circuiting A condition that occurs in tanks or basins when some of the water travels faster than the rest of the flowing water. This is usually undesirable since it may result in shorter contact, reaction, or settling times in comparison with the theoretical (calculated) or presumed detention times. Surface Loading One of the guidelines for the design of settling tanks and clarifiers in treatment plants. Used by operators to determine if tanks and clarifiers are hydraulically over or under loaded. Also called overflow rate. Septic A condition produced by anaerobic bacteria. If severe, the wastewater produces hydrogen sulfide, turns black, gives off foul odors, contains little or no dissolved oxygen, and the wastewater has a high oxygen demand. Purpose of Primary Treatment To slow incoming wastewater velocity to allow removal of settleable organic material as well as floatable material. Process Description The Primary Sedimentation process consists of a large tank or basin that is designed to allow gravity to separate solids from wastewater. Material with a specific gravity higher than water will sink and material with a lower specific gravity will float. The materials that float and settle are removed from the primary clarifier and sent to a solids handling process. The clarified water moves along to the next process for further treatment. This is usually accomplished with a Detention Time of 2 3 hours. - 11 -
Component Influent control gate Influent pipe/channel Target baffle/deflector Effluent weir Effluent trough Sludge collector Sprocket Angle track Sludge collector drive Cross collector Sump Sludge pump Wearing shoe Scum skimmer Scum trough Scum baffle Purpose Controls flow into clarifier Transports wastewater to clarifier Distributes wastewater evenly in clarifier (prevents short circuiting) Ensure equal flow over all weirs Collects and transports settled water to next process Moves settled sludge to the sludge sump Supports sludge collector chain and allows for tension adjustment Provides a track for the main sludge collector crosspieces to ride on Provides the power for the sludge collector (main and cross collectors) Moves sludge to the deep end of the sump for removal Collection point for settled sludge Removes sludge from sump Prevents wear on the sludge collector crosspieces Moves scum and other floating material to the scum trough Receives floating material and scum for removal Prevents scum and floating material from reaching effluent - 12 -
Component Influent control gate Influent channel Influent well Effluent weir Effluent trough (launder) Scum skimmer Scum trough Scum pipe Drive unit Vertical drive cage Sludge collector Blades and scraper Sump Sludge pipe Controls flow to clarifier Transports wastewater to clarifier Purpose Reduces flow velocity and distributes flow evenly Ensures equal flow over all weirs Collects and transports settled water to next process Moves scum and other floating material to the scum trough Receives floating material and scum for removal Allows collected scum to be removed from scum trough Provides power for sludge collector rotation Transfers power from drive unit to sludge collector Drags settled sludge across clarifier bottom to sludge sump On bottom of sludge collector to scrape sludge on clarifier bottom Collects settled sludge Removes sludge for further processing (digestion) - 13 -
Water Quality Indicator Typical Removal Efficiencies Typical Removal Efficiency Settleable Solids 95% - 99% Suspended Solids 40% - 60% Total Solids 10% - 15% Biochemical Oxygen Demand (BOD) 20% - 50% Bacteria 25% - 75% Sludge Pumping 1. Sludge should be as thick as possible 2. Solids should be between 4% - 8% as determined by Total Solids Test 3. Pump before it goes septic Safety There are several dangerous gases present around wastewater treatment plants. Operators should be aware of the following: Hydrogen Sulfide (H 2 S) Causes a rotten egg odor Causes olfactory fatigue (dulls sense of smell) Extremely toxic to respiratory system Corrosive when mixed with oxygen (H 2 SO 4 sulfuric acid) Flammable and explosive when mixed with proper amount of air Carbon Dioxide (CO 2 ) Odorless and tasteless Can displace oxygen and cause asphyxiation (10% in air can cause death) Carbon Monoxide (CO) Colorless, odorless, does not cause irritation Flammable and explosive Interferes with bloods ability to carry oxygen Causes asphyxiation - 14 -
Methane (CH 4 ) Odorless Flammable and Explosive Can cause asphyxiation Chlorine (Cl 2 ) Green-yellow gas that is heavier than air Irritating to eyes, skin, mucous membrane and lungs Causes death by asphyxiation or lung damage Practice Questions True/False 1. The sludge collectors in round and rectangular clarifiers are the same 2. Solids returned to the primary clarifier from another treatment process will always settle completely 3. It is not possible to have too many clarifiers in operation at one time 4. Primary sludge and Secondary process sludge are very similar in appearance and settling characteristics 5. If sludge is not pumped regularly, a septic condition may develop in the primary sedimentation basin 6. Sludge being pumped to the digester should be as thin as possible to make it easier to pump Multiple Choice 1. What is the main difference between a primary and secondary clarifier? a. Density of water treated b. Density of sludge handled c. Depth of water d. Construction material - 15 -