Wastewater Engineering 2

Wastewater Engineering Module 2

• Microbiology of Sewage
• Unit Operation & Unit Process
• Preliminary Treatment

Microbiology of Sewage

• Microbes play an extremely important role in sewage treatment.
• Sewage is converted from a highly contaminated, infectious liquid into a relatively stable, inert sludge and a harmless effluent through biological digestion.
• The effluent needs only chlorination before it may be discharged into a receiving stream, leaching bed, or other disposal area.
• Microorganisms are significant in water and wastewater because of their roles in disease transmission.
• Domestic wastewater contains enormous quantities of microorganisms.
• Bacterial counts in raw sewage may range from 500,000 to 5,000,000 per mL, depending on sewage age and the quantity of dilution of water.

Bacterial Growth Pattern

• Bacteria are single-celled plants which metabolize soluble food and reproduce by binary fission.

• In the presence of adequate food and a suitable environment, bacteria will grow according to the following phases:

  • Lag Phase: The acclimatization or adjustment phase of microorganisms.
  • Log Growth Phase: Microorganisms multiply since there is an ample supply of food.
  • Declining Growth Phase: The point at which the food is largely depleted and food becomes the limiting factor in further growth.
  • Stationary Phase: Food supply is equal to microorganism population.
  • Endogenous Phase: The total mass of microorganisms will slowly decrease as the cells use up all their stored reserves and slowly begin to die. During this phase, lysis can occur, where nutrients remaining in the dead cells diffuse out to furnish the remaining cells with food (cryptic growth).
  • F/M < 1

• Obligate anaerobes: Restricted to environments without dissolved oxygen.

• Aerotolerant anaerobic bacteria: Can thrive in both ample oxygen and absence of oxygen.

• Nitrification: Some bacteria can survive in nitrogen deficient environments, indicating stabilization of wastewater.

• Filaments: Too many filaments can cause bulky sludge. These are important for pollutant removal using bio microorganisms.

Protozoa

• Protozoa are single-celled eukaryotes that show characteristics usually associated with animals.
• Protozoa are responsible for improving the quality of the effluent, maintaining the density of dispersed bacterial populations by predation.
• In wastewater treatment, protozoa are a critical part of the purification process and can be used to indicate the condition of treatment processes.

Algae

• Algae are plants, many microscopic, containing chlorophyll.
• Freshwater algae are diverse in shape, color, size, and habitat.
• They are the basic link in the conversion of inorganic constituents in water into organic constituents.
• Algae and cyanobacteria typically grow in the aerobic zone and provide bacteria in the pond with plenty of oxygen during the daytime.
• Algal photorespiration may consume oxygen during night-time when it is dark.
• Algae can be used in wastewater treatment for a range of purposes, including:
  • Removal of coliform bacteria
  • Reduction of both chemical and biochemical oxygen demand
  • Removal of N and/or P
  • Removal of heavy metals

Fungi

• Fungi are simple plants lacking the ability to produce energy through photosynthesis.
• Fungi are eukaryotes and have a complex cellular organization.
• Fungi are indicated for their superior aptitudes to produce a large variety of extracellular proteins, organic acids, and other metabolites, and for their capacities to adapt to environmental constraints.

Virus

• Viruses are the smallest form of microorganisms (others classify it as particle and not as microorganism) capable of causing disease.
• Viruses are a major hazard to public health.
• Some viruses can live for 41 days in water and wastewater at 20 °C.
• More attention must be paid to viruses, however, when surface water supplies have been used for sewage disposal.
• Viruses are difficult to destroy by normal disinfection practices, as they require increased disinfectant concentration and contact time for effective destruction.

Rotifers

• Rotifers make up a well-defined group of the smallest, simplest multicellular microorganisms and are found in nearly all aquatic habitats.
• Rotifers are a higher life form associated with cleaner waters.
• Normally found in well-operated wastewater treatment plants, they can be used to indicate the performance of certain types of treatment processes.

Microorganism Preferences

• Generally speaking, microorganisms are like people who prefer:
  • Enough room to move around
  • Just sufficient food
  • Enough nutrients
  • Enough oxygen to breathe

Coliform Organisms

• Coliform Organisms are rod shaped bacteria thriving inside the intestinal tract of man.
• Each person discharges from 100 to 400 billion coliform organisms per day.
• They are harmless to man and are, in fact, useful in destroying organic matter in biological waste treatment processes.
• The presence of coliform organisms is taken as an indication that pathogenic organisms may also be present, and the absence of coliform organisms is taken as an indication that the water is free from disease producing organisms.
• Total Coliform: all aerobic, facultative and anaerobic gram-negative, non-spore forming, rod-shaped bacteria that ferment lactose with gas formation within 48 hours
• Coliform Organisms bacteria include the genera Escherichia and Aerobacter. Other genera: Citrobacter, Hafnia, and Klebsiella
• There is difficulty in determining E.coli to the exclusion of the soil coliforms; as a result, the entire coliform group is used as an indicator of fecal pollution.
• E.coli is the preferred pathogen indicator. Note: this is not the pathogenic E.coli O157:H7 strain. harmless ecoli
  • Properties of E.coli are:
    • Found in much higher concentrations than most pathogens in fecal matter.
    • Non-pathogenic
    • Easy to detect, relatively fast and inexpensive analysis.
    • Its absence indicates absence of enteric pathogens
• Test Procedure for determining the presence of coliform:
  • Presumptive test - based on the ability of the coliform group to ferment lactose broth, producing gas.
  • Confirmed test - consists of growing cultures of coliform bacteria on media that suppress the growth of other organisms.
• Accepted Methods for determining the number of Coliform organisms present in a given volume of water:
  • Most Probable Number (MPN) – has been used for a long time and is based on a statistical analysis of the number of positive and negative results obtained when testing multiple portions of equal volume and in portions constituting a geometric series for the presence of coliform.
    • Note: MPN is not an absolute concentration of organisms that are present.
  • Membrane Filter Technique (MFT) - is accomplished by passing a known volume of water sample through a membrane filter that has a very small pore size. The bacteria are retained on the filter. The bacteria are then contacted with an agar that contains necessary nutrients necessary for growth of the bacteria. After incubation, the coliform colonies can be counted and the concentration in the original water sample is determined.
    • Faster than MPN and gives a direct count of the number of coliforms
• Fecal Coliforms (FC) vs. Fecal Streptococci (FS)
  • It has been observed that the quantities of fecal coliforms and fecal streptococci that are discharged by human beings are significantly different from the quantities discharged by animals.
  • Sampling Techniques:
    • Objectives of Sampling:
      • Evaluate performance efficiency of WTP
      • Compliance to DENR requirements

• Grab Sampling - sample is taken at random with no particular time.
  • It may be taken from the discharge of a pump, be manually dipped from the flow, or be automatically dipped or siphoned from the stream.
• Composite Sampling - is a mixture of grab samples taken over a period of time, with the volume of individual samples usually being proportional to the flow at the time the sample is taken.
  • Composite samples are most useful for analyses of average characteristics such as daily waste loads
• Continuous Sampling - sample represents diversion of a small fraction of the total flow over some period of time.
  • Continuous samplers are usually not flow proportional. Rather, they extract the sample at a constant rate.
  • Continuous samplers are most suitable for instrumental measurements which can be performed virtually instantaneously, such as temperature, pH, DO, etc.
• Important Notes on Sampling:
  • Examination of drawings that shows sewers and manholes will help to determine sampling locations where flow conditions encourage a homogenous mixture.
  • However, if the discharger intends to evaluate the performance efficiency of the whole WTP, samples must be taken from influent stream to the WTP and from the effluent stream of each treatment unit. Effluent from specific plant operation/process or production area must be taken to evaluate effectiveness of in-plant water pollution control or waste minimization measure/s.
  • In sewers and in deep, narrow channels, samples should be taken from a point 1/2 ord the water depth from the bottom. The collection point in wide channels should be rotated across the channel.
  • Samples must be suitably preserved until they can be analyzed. Domestic sewage samples can be preserved satisfactorily by storage at 4°C. Freezing alters the character of the solids and thus should be avoided.
• Composite Sampling Procedure and Calculation Procedure:
  • Determine the total amount of sample required for all tests to be performed on the composite sample.
  • Determine the average daily flow of the treatment system. Average daily flow can be determined by using several months of data will provide a more representative value.
  • Calculate a proportioning factor:

Preliminary Treatment

• Screening:
  • Coarse screen (6 to 150mm)
  • Fine screen (< 6mm)
  • Microscreen (< 50μm)
    • Hand cleaned
    • Mechanically cleaned
      • Static wedgewire
      • Drum type
      • Step type
      • Chain driven
      • Reciprocating rake
      • Continuous belt
• Flow Measurement: Flow rate needed for efficient operation, chemical addition, etc.
  • Several operations need flow rate data for good operations (i.e. Chlorination, pH adjustment, etc.)
  • Legislative requirement
  • Various Types of WW Flowrate used in Process design:
    • Average Daily Flowrate (ADF): average flowrate over a period of time
    • Peak Hourly Flowrate (PHF): highest flowrate measured in 24-h period.
    • Maximum Daily Flowrate (MDDF): maximum daily design flow over a period of time.

• Turndown ratio - the ratio of the highest operating flow rate to the lowest operating flow.
• Solids grinding or comminution - grinding of coarse solids to a more or less uniform size.
• Comminutors - devices that used to cut up the solids in wastewater.
• Barminutor - is a combination of a bar screen and a comminutor. The bar screen traps the rags and a rotating cutter runs down the screen to cut up the rags every 15-30 minutes.
• Grit Removal
  • Grit Chamber - is an enlarged channel where the velocity of wastewater flow is controlled to allow only the heavier solids to settle out.
  • The downward slope of a sewer line must be sufficient to maintain a minimum velocity of 2 feet per second in the pipe. This velocity is maintained until the flow enters the treatment plant. The grit is removed from the flow as it passes through a grit chamber. The velocity in the grit chamber is reduced to about 1 foot per second.
  • Variation types: (a) Horizontal-flow grit chamber (b)Aerated grit chamber (c) Circular or Vortex type
• Flow Equalization
  • Equalization basin - maintains a constant volumetric flow of wastewater from pretreatment to other downstream operation. It dampens hydraulic or flowrate variations.
  • Specific Objectives of Equalization:
    • Smooth out fluctuations in flow rate
    • Dampens the variation in the concentration of BOD5, and TSS.
    • Decrease fluctuations in flow rate, to provide more consistent treatment. Done by storing excess wastewater during high flow periods. Results in more consistent treatment.

Computations on Preliminary Treatment

Screening Removal Calculations

• Conversions: 1 m^3 = 7.48 gal
• To keep and maintain accurate screening records, the volume of screenings withdrawn must be determined.
• Screening removal rate = \frac{volume \, of \, screenings \, removed}{day}

Screening Pit Capacity Calculations

• In screening pit capacity problems, the time required to fill a screening pit is calculated. The equation used in screening pit capacity problem is given below:
• screening \, pit \, filling \, time (day) = \frac{volume \, of \, pit (cubic \, ft)}{screening \, removal \, rate (cu.ft/day)}

Grit removal, Grit channel velocity and Settling time

• Grit \, removal \, (\frac{cu.ft}{MGD}) = \frac{volume \, of \, grit \, removed (cu. ft)}{plant \, flow \, rate (MGD)}
• Grit \, velocity = \frac{influent \, flow \, rate}{No. \, of \, channels \, in \, service \, x \, channel \, width \, x \, water \, depth}
• settling \, time = \frac{liquid \, depth}{settling \, velocity}