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Quiz & test questions Grade 11 Careers & CTE

Hollenmarsh Digester Operations: A Biosolids Operator Quiz

A grade 11 careers quiz on running anaerobic digesters at a water resource recovery facility, covering volume, detention time, solids loading, and biogas.

Objectives

  1. Students will calculate digester volume, detention time, and solids loading using standard operator formulas.
  2. Students will explain why an anaerobic digester is held near a set temperature and what that heat does not accomplish.
  3. Students will describe two routes a person can take into biosolids and resource recovery work.

Vocabulary

Anaerobic digester
An anaerobic digester is a sealed tank where bacteria break down sewage sludge without any oxygen, shrinking its volume and producing methane gas as a byproduct. Some treatment plants capture that gas and burn it to generate their own electricity or heat.
Detention time
Detention time is how long water stays inside a tank or basin before flowing out. Water must stay long enough for each treatment step, like settling or disinfecting, to finish its job.
Biosolids
Biosolids are the nutrient-rich solid material left over after sewage sludge has been treated and made safe. Many farms use treated biosolids as fertilizer for their fields.

Also: volatile solids, biogas, resource recovery

Teacher overview

This 45-minute quiz is worth 35 points and checks whether students understand the work of the operator who runs anaerobic digesters at a water resource recovery facility. It opens with a multiple-choice item and a short-answer item, works through eight computation items drawn from operator certification practice, and closes with a question about how people enter this part of the field. The plant is a fictional facility in the town of Hollenmarsh. One misconception is worth settling before students begin, because many assume the digester is heated in order to kill pathogens. The heat holds the methane-forming microorganisms near the temperature where they work steadily, and a digester that swings in temperature loses gas production and can sour. Pathogen reduction is a separate requirement, met by holding the sludge for a required time at a required temperature or by further processing, so a warm digester by itself does not make biosolids safe to land apply. Items 2 and 11 rest on that distinction.

Hollenmarsh Digester Operations: A Biosolids Operator Quiz — Grade 11 Careers & CTE
Name: ____________________________ Date: ______________

Assessment

  1. (2 points) At a water resource recovery facility, what does the operator assigned to the digesters mainly do?
    • A) Sets the sewer rates the utility will charge next year
    • B) Controls feed, temperature, and mixing so the digester stays stable, and tracks gas and solids results
    • C) Issues the discharge permit the plant operates under
    • D) Designs the new headworks building
  2. (3 points) The digester is held near a set temperature all year. Explain why that heat is needed. Then explain why heating alone does not make the biosolids safe to land apply.
  3. (4 points) The fictional Hollenmarsh digester is a cylinder. It measures 50 feet across, and sludge stands 24 feet deep in it. Find its volume in gallons. Use 0.785 times the diameter squared times the depth for cubic feet, then use 7.48 gallons per cubic foot.
  4. (4 points) A digester holds 352,308 gallons. It is fed 15,000 gallons of sludge each day. Find the detention time in days. Round to the nearest tenth.
  5. (4 points) A digester is fed 15,000 gallons per day, which is 0.015 MGD. That sludge is 4.2 percent total solids, and one percent equals 10,000 mg/L. Find the pounds of total solids fed each day, using pounds per day equal to concentration in mg/L times flow in MGD times 8.34. Round to the nearest pound.
  6. (3 points) A digester receives 5,254 pounds of total solids each day. Volatile solids make up 72 percent of those solids. Find the pounds of volatile solids fed each day. Round to the nearest pound.
  7. (3 points) A digester is fed 3,783 pounds of volatile solids each day. The sludge drawn off carries 1,550 pounds of volatile solids each day. Find the percent of volatile solids destroyed. Round to the nearest tenth.
  8. (3 points) A digester destroys 2,233 pounds of volatile solids each day. The fictional plant's records show 12 cubic feet of biogas for every pound destroyed. Find the biogas produced each day, in cubic feet.
  9. (3 points) A digester produces 26,796 cubic feet of biogas each day. The plant's engine makes 5.0 kWh of electricity for every 100 cubic feet of biogas it burns. Find the electricity produced each day. Round to the nearest kilowatt-hour.
  10. (3 points) The fictional Hollenmarsh plant treats 3.2 MGD. Using the rate that one MGD equals 694 gpm, convert that flow to gallons per minute. Round to the nearest gallon per minute.
  11. (3 points) Name two different routes a person could take into digester and biosolids work at a treatment plant. State one thing each route would require.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: B. The digester operator manages feed volume, temperature, and mixing, then reads gas production and solids results to judge whether the process is stable. Rates belong to the finance office, permits to the state agency, and building design to a consulting engineer.
  2. Answer: The heat keeps the methane-forming microorganisms near the temperature at which they digest steadily, so gas production holds and the digester does not sour when loading changes. Heating alone does not make the biosolids safe to land apply, because pathogen reduction is judged separately, by holding the sludge for a required time at a required temperature or by further processing before it leaves the plant. Accept any answer that pairs the heat with a stable microbial population and states that pathogen reduction is a separate requirement.
  3. Answer: 352,308 gallons. Volume in cubic feet is 0.785 times 50 squared times 24. Since 50 squared is 2,500, the product is 0.785 times 2,500 times 24, which equals 47,100 cubic feet. Multiplying 47,100 by 7.48 gallons per cubic foot gives 352,308 gallons.
  4. Answer: 23.5 days. Detention time is volume divided by flow, so 352,308 divided by 15,000 equals 23.487, which rounds to 23.5 days. An operator would report this as a little over three weeks of digestion.
  5. Answer: 5,254 lbs/day. Total solids at 4.2 percent equal 4.2 times 10,000, or 42,000 mg/L.
    Pounds per day is 42,000 times 0.015 times 8.34. Since 42,000 times 0.015 equals 630, the product is 630 times 8.34, which equals 5,254.2 pounds per day.
  6. Answer: 3,783 lbs/day. Volatile solids are 72 percent of the total, so 5,254 times 0.72 equals 3,782.88, which rounds to 3,783 pounds per day.
  7. Answer: 59.0 percent. The mass destroyed is 3,783 minus 1,550, or 2,233 pounds per day.
    Dividing 2,233 by 3,783 gives 0.5903, which is 59.0 percent. A conventional digester running well is usually judged against a target in this range.
  8. Answer: 26,796 cu ft. Multiplying 2,233 pounds destroyed by 12 cubic feet per pound gives 26,796 cubic feet of biogas per day.
  9. Answer: 1,340 kWh. There are 26,796 divided by 100, or 267.96 hundred-cubic-foot units of gas. Multiplying 267.96 by 5.0 kilowatt-hours gives 1,339.8, which rounds to 1,340 kilowatt-hours per day.
  10. Answer: 2,221 gpm. Multiplying 3.2 by 694 gallons per minute per MGD gives 2,220.8, which rounds to 2,221 gallons per minute.
  11. Answer: Accept any two distinct routes with a matching requirement. Reasonable answers include entering as a plant helper or maintenance worker and earning a state wastewater operator license by exam after documented experience hours; completing a two-year wastewater technology or environmental technology program and testing into an entry license class; transferring from a drinking water license into wastewater by passing the separate wastewater exam; arriving through an agricultural or soil science background into the land application and nutrient management side of biosolids; and moving up from a laboratory technician position while accumulating operating hours. Each route is a legitimate way into the work.

Differentiation

Support

Give students a formula card listing the cylinder volume formula, the gallons per cubic foot conversion, detention time as volume divided by flow, and the pounds formula, each with one worked example on a different fictional digester. Allow a calculator throughout. Let item 2 be answered as a labeled cross-section sketch with two captions rather than a paragraph.

Extension

Ask students to recalculate item 4 if the fictional plant raises the feed from 15,000 to 18,000 gallons per day. The detention time falls to 19.6 days. Have them state one thing an operator would watch after that change, such as gas production, volatile acids, or the alkalinity ratio, and explain why a shorter detention time makes it worth watching.

ELL supports

Pre-teach "digester," "volatile solids," "detention time," "biosolids," and "biogas" with a labeled cross-section drawing that shows feed in, gas out, and sludge withdrawn. Provide a sentence frame for item 2: "The heat keeps the ___ working steadily, but the biosolids are not safe to land apply until ___." Allow item 11 to be answered as a list rather than sentences.

Extensions

Have students compare the electricity from item 9 against a daily plant electricity demand that the teacher supplies, or that students find for a real plant treating about the flow in item 10, and decide what share of plant power the digesters could cover, then write two sentences on what that means for the operator's incentive to keep the process stable. Students who want to follow the career can look up whether their state issues a separate credential for land application or biosolids management, list what it requires, and compare that with the wastewater operator license their state issues for plant work.