Worksheet Grade 8 Science

Pellmarsh's Digester: Solids, Methane, and Plant Energy

A grade 8 science worksheet on a fictional plant's anaerobic digester: volatile solids destroyed, biogas produced, and yearly electricity in scientific notation.

Grade: Grade 8
Subject: Science
Material: Worksheet
Time: 35 min
Prep: No prep
Materials: Printed worksheet, Calculators

Standards

MS-ESS3-3 NGSS

Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.

How this material aligns: Item 10 has students evaluate methane capture as a way to reduce the plant's impact, using the electricity and nitrogen figures they calculated in items 6, 7, and 9, and the extension weighs that use of the gas against cleaning it up for vehicle fuel.

All materials for MS-ESS3-3 →

8.EE.A.3 Common Core Math

Use numbers expressed in the form of a single digit times an integer power of 10 to estimate very large or very small quantities, and to express how many times as much one is than the other.

How this material aligns: Items 4 and 7 have students write the daily biogas volume and the yearly electricity output in scientific notation as a power of ten.

All materials for 8.EE.A.3 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will calculate the volatile solids an anaerobic digester destroys and express that loss as a percent.
Students will convert daily biogas volume and yearly electricity output into scientific notation.
Students will explain how capturing methane and reusing biosolids reduces a treatment plant's impact.

Vocabulary

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: anaerobic digestion, volatile solids, biogas, methane, kilowatt-hour

Teacher overview

A wastewater plant does more than clean water. At the fictional Pellmarsh Water Resource Recovery Facility, the thick sludge separated from the wastewater goes into a sealed tank called a digester, where bacteria that live without oxygen break the solids down and give off a gas that is mostly methane. Pellmarsh burns that gas in an engine to make electricity for the plant, and it spreads the leftover solids on farm fields as fertilizer. Students often assume digestion is the same as composting; composting needs oxygen, while the bacteria in a digester only work when oxygen is kept out, which is why the tank is sealed.

Pellmarsh's Digester: Solids, Methane, and Plant Energy — Grade 8 Science

Name: ____________________________ Date: ______________

Student Worksheet

All figures below come from the fictional Pellmarsh Water Resource Recovery Facility. Show your work for every calculation.

Pellmarsh feeds 3,600 pounds of volatile solids into its digester each day. Measurements show 1,440 pounds leaving the digester still undigested. How many pounds of volatile solids do the bacteria destroy each day?
Using the same figures (3,600 pounds in and 1,440 pounds out), what percent of the volatile solids are destroyed?
Pellmarsh's digester makes 15 cubic feet of biogas for every pound of volatile solids destroyed. Using the 2,160 pounds destroyed each day, how many cubic feet of biogas does the digester make in one day?
Write 32,400 cubic feet of biogas in scientific notation.
Pellmarsh's biogas is 62 percent methane by volume. How many cubic feet of methane are in 32,400 cubic feet of biogas?
The plant's engine makes 6.0 kilowatt-hours of electricity for every 100 cubic feet of biogas it burns. How many kilowatt-hours does 32,400 cubic feet of biogas produce in one day?
At a rate of 1,944 kilowatt-hours per day, how many kilowatt-hours would the engine make in 365 days? Write the answer in scientific notation, rounded to three significant figures.
Explain why the bacteria in the digester must work without oxygen, and name the gas they release that the plant burns.
One day's digested solids from Pellmarsh carry 210 pounds of nitrogen to nearby farm fields. If a farmer applies 60 pounds of nitrogen per acre, how many acres does one day's solids fertilize?
Using your answers about biogas and electricity, explain in two or three sentences how capturing methane at Pellmarsh reduces the plant's effect on the environment.

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

Answer Key

Answer: 2,160 pounds.
3,600 pounds in minus 1,440 pounds out leaves 2,160 pounds of volatile solids destroyed each day.
Answer: 60 percent.
2,160 divided by 3,600 equals 0.6, and 0.6 times 100 gives 60 percent destroyed. Accept work shown as a fraction reduced to 3/5.
Answer: 32,400 cubic feet.
2,160 pounds destroyed times 15 cubic feet per pound equals 32,400 cubic feet of biogas per day.
Answer: 3.24 x 10^4 cubic feet.
Move the decimal point four places to the left, from 32,400 to 3.24, so the exponent is 4.
Answer: 20,088 cubic feet.
0.62 times 32,400 equals 20,088 cubic feet of methane. A student who writes 62/100 times 32,400 should reach the same value.
Answer: 1,944 kilowatt-hours.
32,400 divided by 100 equals 324 hundred-cubic-foot units, and 324 times 6.0 equals 1,944 kWh in one day.
Answer: 7.10 x 10^5 kilowatt-hours.
1,944 times 365 equals 709,560 kilowatt-hours per year. Rounded to three significant figures that is 710,000, or 7.10 x 10^5.
Answer: The bacteria are anaerobic, and they release methane.
These bacteria break down solids only when oxygen is absent, so the digester is sealed. The methane they give off is the fuel Pellmarsh burns in its engine.
Answer: 3.5 acres.
210 pounds of nitrogen divided by 60 pounds per acre equals 3.5 acres fertilized by one day's solids.
Answer: Capturing methane replaces purchased electricity and keeps the gas out of the air.
Methane released to the atmosphere is a strong heat-trapping gas, so burning it in the engine keeps it out of the air. The 1,944 kilowatt-hours per day the engine makes is electricity Pellmarsh does not have to buy, and the digested solids return nitrogen to farm fields instead of going to a landfill.

Differentiation

Support

Give students a completed model of item 1 and item 2 using different figures, such as 2,000 pounds in and 800 pounds out, worked all the way through the subtraction and the percent. Provide a place-value strip for items 4 and 7 so students can count decimal places before writing the exponent.

Extension

Ask students to find how many days of digester gas the fictional Pellmarsh plant would need to run a 500 kilowatt-hour classroom load for a month, then write a short recommendation about whether the plant should burn the gas for electricity or clean it up for use as vehicle fuel.

ELL supports

Pre-teach "digester," "volatile solids," and "biogas" with a labeled sketch showing sludge entering a sealed tank, gas leaving the top, and solids leaving the bottom. Provide sentence frames such as "The bacteria destroyed ___ pounds because ___." Allow students to answer items 8 and 10 with a labeled diagram plus one sentence.

Extensions

Have students compare the fictional Pellmarsh figures with a published biosolids report from a nearby city and list two questions they would ask an operator about how the plant uses its digester gas.