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NCERT Solutions for CBSE Class 7 Science Chapter 13: Wastewater Story

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CBSE Class 7 Science Chapter 13 Wastewater Story is a cornerstone chapter in the Biology and Environmental Science unit, introducing students to the journey of water after it leaves our homes. Every flush, every shower, every kitchen rinse generates wastewater—also called sewage—that must be treated before returning to rivers or soil. This chapter walks students through the composition of sewage, the multi-stage treatment process in municipal plants, the health hazards of untreated wastewater, and simple practices every household can adopt to reduce pollution. Mastery of these NCERT solutions is critical not only for scoring in Class 7 internal assessments but also for building foundational understanding of sanitation, a recurring theme in CBSE Classes 8, 9, and even Class 10 Board exams.

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Key takeaways

✓CBSE Class 7 Science Chapter 13 Wastewater Story covers sewage composition, treatment processes, and sanitation—essential topics for environmental awareness and board exams.
✓Wastewater comprises organic impurities, suspended solids, nutrients, and harmful bacteria; untreated sewage leads to waterborne diseases like cholera and typhoid.
✓Sewage treatment involves physical removal of large objects, bacterial decomposition in aeration tanks, and sludge digestion to produce biogas and safe effluent.
✓The chapter teaches the difference between sewage and sludge, the role of aerobic versus anaerobic bacteria, and the importance of on-site sanitation systems like septic tanks.
✓Better housekeeping practices—avoiding oil, chemicals, and solid waste disposal in drains—protect both home plumbing and public treatment infrastructure.
✓Questions from CBSE Class 7 Science Chapter 13 Wastewater Story frequently appear in CBSE unit tests and annual exams, typically carrying 2-5 marks each.
✓CBSETUTOR.ai provides 24×7 AI-powered doubt-solving for every NCERT question in Class 7 Science, including photo uploads of diagrams for instant, step-by-step guidance at ₹999/month.

Overview of CBSE Class 7 Science Chapter 13 Wastewater Story

CBSE Class 7 Science Chapter 13 Wastewater Story is part of Unit 4 (Water) in the 2024-25 NCERT syllabus. The chapter spans approximately 10 pages and is divided into four major sections: sources and composition of sewage, the workings of wastewater treatment plants, the link between sanitation and disease, and better housekeeping practices. Students encounter terms like sewage, sludge, clarifier, aeration tank, activated sludge, and biogas for the first time. The chapter emphasizes both scientific concepts—bacterial action, aerobic versus anaerobic decomposition—and civic responsibility. Diagrams of treatment plant layouts and cross-sections of septic tanks are integral to the NCERT text and often appear in CBSE exam questions. Teachers typically allocate 4-5 periods to this chapter, with at least one practical demonstration or video of a sewage treatment plant. Questions from CBSE Class 7 Science Chapter 13 Wastewater Story carry 8-12 marks in the annual exam, distributed across MCQs, short answers, and one long answer.

Chapter length: approximately 10 pages in NCERT textbook
Four main topics: sewage composition, treatment plants, sanitation-disease linkage, household practices
Key vocabulary: sewage, sludge, clarifier, aeration, activated sludge, biogas, vermi-processing, septic tank
Typical weightage: 8-12 marks in CBSE Class 7 annual exam
Assessment pattern: 2-3 MCQs (1 mark each), 2-3 short answers (2-3 marks), 1 long answer (5 marks)

What is Sewage? Understanding Composition and Sources

Sewage is wastewater released by homes, industries, hospitals, offices, and agricultural fields. It is approximately 99.9% water by weight; the remaining 0.1% consists of suspended solids, organic matter (human waste, food residues, soaps, detergents), inorganic impurities (sand, grit, phosphates), nutrients (nitrogen, phosphorus), and harmful microorganisms (bacteria, viruses, protozoa). The NCERT text in CBSE Class 7 Science Chapter 13 Wastewater Story clarifies that untreated sewage is a breeding ground for pathogens causing cholera, typhoid, dysentery, and hepatitis. Domestic sewage includes water from toilets (blackwater) and kitchens, bathrooms, laundries (greywater). Industrial effluents may contain heavy metals, chemicals, and heat. Agricultural runoff contributes fertilizers and pesticides. Students must be able to list sewage components and explain why each poses environmental or health risks—a frequent 3-mark question in CBSE exams.

Sewage = 99.9% water + 0.1% impurities (organic, inorganic, microbial)
Organic impurities: human excreta, food waste, paper, soaps, detergents, urea
Inorganic impurities: sand, grit, metal salts, phosphates, nitrates
Microbial load: bacteria (E. coli), viruses, protozoa, worm eggs
Sources: households, industries, hospitals, farms, stormwater drains

Step-by-Step Wastewater Treatment Process

The NCERT textbook in CBSE Class 7 Science Chapter 13 Wastewater Story describes a three-stage municipal sewage treatment process. Stage 1 (Physical Treatment): Wastewater passes through bar screens that remove large floating objects (plastic bags, sticks, rags). It then enters a grit and sand removal tank where heavy particles settle by gravity. Stage 2 (Primary Treatment): Water flows into a primary settling tank (clarifier) where suspended solids settle as primary sludge; lighter materials like oil and grease float and are skimmed off. Stage 3 (Secondary Treatment / Biological Treatment): Clarified water enters an aeration tank where air is pumped in continuously. Aerobic bacteria consume organic matter, forming activated sludge. This mixture moves to a secondary settling tank; clear water (effluent) is discharged into rivers or used for irrigation, while activated sludge is partly recycled and partly sent to a digester. In the digester, anaerobic bacteria decompose sludge, producing biogas (methane-rich, used for energy) and stable residue used as manure. Students must draw and label a flowchart of this process—a standard 5-mark question.

Bar screens → remove large floating debris (sticks, plastics, cloth)
Grit removal tank → settle sand, gravel by gravity
Primary clarifier → suspended solids settle as primary sludge; oil/grease skimmed
Aeration tank → aerobic bacteria decompose organic matter; continuous air supply
Secondary clarifier → activated sludge settles; clear water released
Anaerobic digester → sludge decomposed by bacteria in absence of oxygen; produces biogas and manure

Role of Microorganisms in Sewage Treatment

CBSE Class 7 Science Chapter 13 Wastewater Story emphasizes that bacteria are the heroes of wastewater treatment. Aerobic bacteria require oxygen and decompose organic matter rapidly in the aeration tank, converting it into carbon dioxide, water, and microbial biomass (activated sludge). Common aerobic bacteria include species of Pseudomonas and Bacillus. Anaerobic bacteria work in the absence of oxygen inside the digester, breaking down sludge into simpler compounds and releasing methane, carbon dioxide, and hydrogen sulfide (biogas). These bacteria include methanogens like Methanobacterium. Students often confuse the two: aerobic treatment happens in the aeration tank with air pumps running, while anaerobic digestion occurs in closed tanks. A typical CBSE question: 'Differentiate between aerobic and anaerobic bacteria in sewage treatment.' Marks: 3. Another asks: 'Why is air continuously pumped into the aeration tank?' Answer: to supply oxygen for aerobic bacteria to decompose organic impurities efficiently.

Aerobic bacteria: need oxygen; work in aeration tank; decompose organic matter into CO₂, H₂O, biomass
Anaerobic bacteria: work without oxygen; digest sludge in closed digester; produce biogas (CH₄, CO₂, H₂S)
Activated sludge: a mixture of aerobic bacteria and decomposed organic matter; partly recycled to aeration tank
Biogas: methane-rich gas from anaerobic digestion; used for cooking or electricity in treatment plants
Manure: stabilized, dried sludge residue; safe for agricultural use

What Happens to Sludge? Biogas and Manure Production

Sludge is the semi-solid material that settles at the bottom of clarifiers during primary and secondary treatment. It contains high concentrations of organic matter, bacteria, and pathogens. The NCERT text in CBSE Class 7 Science Chapter 13 Wastewater Story explains that sludge is transferred to large closed tanks called digesters where anaerobic bacteria decompose it over several days. The decomposition produces biogas—a mixture of 50-70% methane, 30-40% carbon dioxide, and traces of hydrogen sulfide and other gases. This biogas is captured and used to generate electricity or heat within the treatment plant, making the facility partially energy self-sufficient. The remaining solid residue, after digestion, is dried and converted into nutrient-rich manure used by farmers. Some advanced plants employ vermi-processing, where earthworms further decompose dried sludge, enhancing its quality as compost. Students must explain the dual benefit: energy recovery and safe disposal of organic waste—a recurring 3-mark question in CBSE exams.

Sludge = settled solid waste from primary and secondary clarifiers
Anaerobic digester: closed tank; bacteria break down sludge without oxygen
Biogas composition: ~60% methane (CH₄), ~40% CO₂, trace H₂S
Uses of biogas: cooking, electricity generation at treatment plants
Dried sludge: safe, odorless manure rich in nitrogen and phosphorus
Vermi-processing: earthworms decompose dried sludge into high-quality compost

On-Site Sanitation: Septic Tanks and Their Function

Not all wastewater reaches centralized treatment plants. In rural areas and smaller towns, many homes rely on on-site sanitation systems, primarily septic tanks. A septic tank is an underground chamber made of concrete, fiberglass, or plastic, typically 1-3 meters deep. Wastewater from toilets and bathrooms flows into the tank. Heavy solids settle at the bottom, forming sludge; lighter particles like oil and grease float as scum. In the middle, relatively clear water remains. Anaerobic bacteria in the tank decompose organic matter slowly. The clarified liquid overflows into a soak pit or leach field—a pit filled with gravel where water percolates into the soil. Septic tanks must be desludged every 1-3 years by municipal vacuum trucks. CBSE Class 7 Science Chapter 13 Wastewater Story includes a labeled diagram of a septic tank, and students are often asked to draw and explain it in exams (5 marks). The chapter warns that chemicals, oils, and sanitary napkins must not be flushed into septic tanks as they kill beneficial bacteria and clog the system.

Septic tank: underground chamber for on-site wastewater treatment
Three layers: bottom sludge, middle clear water, top scum (oil, grease)
Anaerobic bacteria decompose organic waste slowly (no air supply)
Overflow: clarified water flows to soak pit; percolates into soil
Desludging: required every 1-3 years; sludge removed by vacuum trucks
Do NOT flush: chemicals, medicines, sanitary napkins, plastics, oil—these harm bacteria and clog pipes

Sanitation and Disease: The Critical Link

Inadequate sanitation is a leading cause of disease and death, particularly in developing countries. CBSE Class 7 Science Chapter 13 Wastewater Story stresses that untreated sewage contaminates drinking water sources, spreads waterborne diseases, and creates breeding grounds for mosquitoes and flies. Diseases caused by poor sanitation include cholera (Vibrio cholerae), typhoid (Salmonella typhi), dysentery (Shigella and Entamoeba), hepatitis A, and parasitic worm infestations. Children under five are especially vulnerable. The chapter cites data showing that access to proper toilets and sewage treatment dramatically reduces infant mortality and school absenteeism. Open defecation—still practiced by millions in India—contaminates soil and water, perpetuating a cycle of disease. Students must be able to list at least three waterborne diseases and explain how sewage treatment prevents their spread—a standard 3-mark question. The NCERT text also highlights the Swachh Bharat Abhiyan (Clean India Mission), which constructed millions of toilets and promoted sanitation awareness from 2014 onwards.

Waterborne diseases from untreated sewage: cholera, typhoid, dysentery, hepatitis A, polio
Pathogens in sewage: bacteria (E. coli, Salmonella, Vibrio), viruses (hepatitis, polio), protozoa (Giardia, Entamoeba), worm eggs
Open defecation contaminates soil and water; flies and animals spread pathogens to food
Proper sanitation (toilets + sewage treatment) reduces child mortality, improves school attendance
Swachh Bharat Abhiyan: government initiative to end open defecation; built over 100 million toilets since 2014

Better Housekeeping Practices to Reduce Water Pollution

CBSE Class 7 Science Chapter 13 Wastewater Story concludes with practical advice for households. Small daily actions collectively reduce the load on sewage treatment plants and protect the environment. Do not pour cooking oil or fats down the kitchen sink—they solidify in pipes, causing blockages and forming fatty deposits (fatbergs) in sewers. Instead, collect used oil in a container and dispose of it with solid waste. Avoid flushing chemicals, medicines, sanitary napkins, cotton swabs, and plastics down toilets; these items do not decompose and harm treatment processes. Use minimum quantities of detergents and soaps; phosphates in detergents promote algal blooms in water bodies (eutrophication). Install a simple mesh or net in kitchen sinks to catch food particles. Do not wash vehicles or dispose of pesticides near drains. Compost kitchen waste at home rather than letting it enter the sewage system. These practices not only ease the burden on treatment infrastructure but also reduce maintenance costs and environmental pollution. Exam questions often ask: 'List four better housekeeping practices to reduce wastewater pollution' (2 marks).

Never pour used cooking oil into sinks—collect and dispose with solid waste
Do not flush medicines, sanitary napkins, plastics, or chemicals—they clog pipes and kill bacteria
Use minimal detergents and soaps; excess phosphates cause eutrophication in rivers
Install sink nets to catch food particles; dispose with compost, not down drains
Avoid washing cars near stormwater drains; detergents pollute water bodies
Compost kitchen waste at home—reduces organic load in sewage

NCERT In-Text Questions and Answers for Chapter 13

The NCERT textbook for CBSE Class 7 Science Chapter 13 Wastewater Story includes in-text questions scattered throughout the chapter to check concept understanding as students read. These are distinct from the end-of-chapter exercises and are often overlooked during revision, yet they hold exam relevance. For instance, one in-text question asks: 'Why should we not throw cooking oil and fats down the drain?' Answer: Cooking oil and fats solidify in pipes, forming blockages and creating a layer on water surfaces that harms aquatic life and treatment bacteria. Another asks: 'What is sewer? How is it different from sewage?' Answer: A sewer is the underground pipe network that carries sewage from homes to treatment plants; sewage is the wastewater itself. A third question: 'Name two chemicals used to disinfect water.' Answer: Chlorine and ozone. Students should attempt these questions first before moving to the end-of-chapter exercise, as they build foundational clarity. Teachers often set class tests with 1-2 in-text questions (1 mark each).

In-text Q1: Why not pour oil down drains? A: Forms blockages, harms aquatic life and bacteria.
In-text Q2: Difference between sewer and sewage? A: Sewer is the pipe network; sewage is the wastewater.
In-text Q3: Two disinfectants for water? A: Chlorine and ozone.
In-text Q4: What is activated sludge? A: Mixture of aerobic bacteria and decomposed organic matter from aeration tank.
In-text Q5: Why is sanitation important? A: Prevents waterborne diseases, protects public health, improves quality of life.

NCERT End-of-Chapter Exercise Solutions for Chapter 13

The end-of-chapter exercise in CBSE Class 7 Science Chapter 13 Wastewater Story comprises 10 questions ranging from fill-in-the-blanks and true/false to short answer (2-3 marks) and long answer (5 marks). Question 1 asks students to fill blanks about cleaning wastewater—terms like 'wastewater', 'sewage', 'clarifier', 'sludge', and 'aerobic bacteria' are tested. Question 2 is true/false, e.g., 'Sewage is mainly liquid waste' (True). Question 3 requires matching processes with treatment stages. Question 4 (2 marks): 'What is sewage? Explain why it is harmful to discharge untreated sewage into water bodies.' Answer: Sewage is liquid waste from homes, industries, etc. Untreated sewage contains pathogens causing diseases; depletes oxygen in water (eutrophication), killing aquatic life. Question 10 (5 marks): 'Describe the steps involved in wastewater treatment. Draw a flowchart.' This long answer must cover physical, primary, secondary, and sludge treatment stages, with a clearly labeled diagram. Detailed solutions to all 10 questions, aligned with the CBSE marking scheme, are essential study material. CBSETUTOR.ai provides instant, step-by-step explanations for every question when students upload their textbook page or write out the question.

Common Mistakes Students Make in Chapter 13 and How to Avoid Them

Many students lose marks in CBSE Class 7 Science Chapter 13 Wastewater Story due to avoidable errors. Mistake 1: Confusing sewage and sludge. Sewage is liquid wastewater; sludge is semi-solid waste that settles during treatment. Mistake 2: Writing that aerobic bacteria work in the digester—aerobic bacteria operate in the aeration tank; anaerobic bacteria work in the digester. Mistake 3: Incomplete flowcharts. A complete wastewater treatment flowchart must show bar screens → grit tank → primary clarifier → aeration tank → secondary clarifier → digester, with clear labels. Missing any step costs marks. Mistake 4: Not drawing diagrams when asked. Questions explicitly stating 'Draw and label' require a neat, labeled diagram; text-only answers receive zero marks for the diagram component. Mistake 5: Using colloquial language like 'dirty water' instead of sewage or wastewater. CBSE marking schemes award marks for precise NCERT terminology. Mistake 6: Forgetting to mention biogas in sludge digestion answers. Always state that anaerobic digestion produces biogas (used for energy) and manure (used as fertilizer).

Confusion: sewage vs. sludge — sewage is liquid waste; sludge is settled semi-solid
Confusion: aerobic vs. anaerobic bacteria — aerobic in aeration tank; anaerobic in digester
Incomplete flowcharts — must include all stages: screening → grit → clarifiers → aeration → digestion
Skipping diagrams when asked — 'draw and label' questions require neat diagrams with clear labels
Using vague language — say 'sewage' or 'wastewater', not 'dirty water'
Forgetting biogas output — always mention biogas and manure as products of sludge digestion

Exam-Focused Tips and Marking Scheme Insights

CBSE Class 7 Science Chapter 13 Wastewater Story typically yields 8-12 marks in the annual examination. The question paper pattern includes 2-3 MCQs (1 mark each), 2-3 short answer questions (2-3 marks), and 1 long answer (5 marks). MCQs test definitions and factual recall—e.g., 'Which gas is produced during anaerobic digestion? (a) Oxygen (b) Methane (c) Nitrogen (d) Hydrogen'—answer (b). Short answer questions ask for lists, differences, or brief explanations: 'List three waterborne diseases.' 'Differentiate between aerobic and anaerobic bacteria.' 'Why is chlorination done before releasing treated water?' Long answer questions require flowcharts, detailed process descriptions, or diagram-based explanations. To maximize marks: (1) Memorize NCERT definitions word-for-word. (2) Practice drawing and labeling diagrams—septic tank, treatment plant flowchart. (3) Use bullet points in answers for clarity. (4) Mention specific examples—E. coli for bacteria, cholera and typhoid for diseases. (5) Write 'aerobic bacteria consume organic impurities in presence of oxygen' rather than 'bacteria clean water'—precision matters. Review previous years' CBSE Class 7 question papers; questions repeat in similar forms.

MCQs (1 mark): definitions, factual recall—e.g., 'What is sludge?'
Short answers (2-3 marks): lists, differences, brief explanations—e.g., 'Why should we not pour oil down drains?'
Long answer (5 marks): flowchart + detailed process description or labeled diagram
Marks for diagrams: neat labels, clear arrows, all components shown
Use NCERT terminology: sewage, sludge, clarifier, aeration, activated sludge, biogas
Time management: allocate 1 min per mark—5-mark question should take 5-7 minutes

How CBSETUTOR.ai Helps Master CBSE Class 7 Science Chapter 13 Wastewater Story

Parents seeking comprehensive support for CBSE Class 7 Science Chapter 13 Wastewater Story can rely on CBSETUTOR.ai, India's first 24×7 AI tutor designed specifically for CBSE Classes 6-12. The platform has ingested every page of the Class 7 NCERT Science textbook, including all diagrams, in-text questions, and end-of-chapter exercises. Students can type or photograph any question from Chapter 13—whether it is 'Draw a labeled diagram of a septic tank' or 'Explain the role of aerobic bacteria in sewage treatment'—and receive instant, step-by-step solutions that mirror CBSE marking schemes. The AI tutor clarifies doubts in simple language, provides additional examples, and even generates practice questions for revision. Unlike generic homework help sites, CBSETUTOR.ai is rooted entirely in NCERT content, ensuring zero contradiction with the official syllabus. It covers all 17 chapters of Class 7 Science, plus Math, Social Science, English, and Hindi for Classes 6-12. The service runs at a flat ₹999 per month—one price for all subjects and classes—with a 3-day free trial that requires no credit card. Parents in cities from Mumbai to Patna use CBSETUTOR.ai as a reliable after-school tutor, ensuring their child never gets stuck on homework or exam prep. For mastering wastewater treatment concepts, diagram practice, and scoring full marks in Chapter 13, CBSETUTOR.ai is an invaluable resource.

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Frequently asked questions

How many marks does CBSE Class 7 Science Chapter 13 Wastewater Story carry in the annual exam?+

CBSE Class 7 Science Chapter 13 Wastewater Story typically carries 8-12 marks in the annual examination, distributed across 2-3 MCQs (1 mark each), 2-3 short answer questions (2-3 marks each), and 1 long answer question with a diagram (5 marks). The exact distribution may vary slightly by year.

What is the difference between sewage and sludge in Chapter 13?+

Sewage is the liquid wastewater discharged from homes, industries, and other sources—it is approximately 99.9% water and 0.1% impurities. Sludge is the semi-solid material that settles at the bottom of clarifiers during treatment—it contains concentrated organic matter, bacteria, and pathogens. Sludge is later digested to produce biogas and manure.

Why is air continuously pumped into the aeration tank during sewage treatment?+

Air is continuously pumped into the aeration tank to supply oxygen to aerobic bacteria. These bacteria require oxygen to rapidly decompose organic impurities in the sewage, converting them into carbon dioxide, water, and microbial biomass (activated sludge). Without oxygen, aerobic bacteria cannot function, and treatment efficiency drops.

What are the main components of biogas produced during sludge digestion?+

Biogas produced during anaerobic digestion of sludge contains approximately 50-70% methane (CH₄), 30-40% carbon dioxide (CO₂), and trace amounts of hydrogen sulfide (H₂S) and other gases. Methane is the valuable component—it burns cleanly and is used for cooking or generating electricity at treatment plants.

How does a septic tank work, and when should it be desludged?+

A septic tank is an underground chamber where wastewater from toilets and bathrooms flows. Heavy solids settle as sludge at the bottom, light materials float as scum on top, and clarified water remains in the middle. Anaerobic bacteria decompose organic waste slowly. Clarified water overflows into a soak pit and percolates into soil. Septic tanks should be desludged every 1-3 years by vacuum trucks.

Which waterborne diseases are caused by untreated sewage?+

Untreated sewage spreads several waterborne diseases including cholera (caused by Vibrio cholerae bacteria), typhoid (Salmonella typhi), dysentery (Shigella and Entamoeba), hepatitis A (virus), and parasitic worm infestations. These diseases result from contaminated drinking water and are especially dangerous for children.

Why should we not pour cooking oil down the kitchen sink?+

Cooking oil and fats solidify in drain pipes, causing blockages and forming large fatty deposits called fatbergs in sewer systems. Oil also creates a layer on water surfaces, reducing oxygen transfer and harming aquatic life and treatment bacteria. Used oil should be collected in a container and disposed of with solid waste.

What is activated sludge in the context of sewage treatment?+

Activated sludge is a mixture of aerobic bacteria and decomposed organic matter formed in the aeration tank during secondary treatment. It is called 'activated' because it is teeming with active bacteria that consume organic impurities. Part of the activated sludge is recycled back to the aeration tank to maintain bacterial population; the rest is sent to the digester.

Can my child get NCERT solutions for all in-text questions in Chapter 13?+

Yes, comprehensive NCERT solutions for CBSE Class 7 Science Chapter 13 Wastewater Story include detailed answers for all in-text questions as well as the end-of-chapter exercise. CBSETUTOR.ai provides instant, step-by-step explanations for every question—students can type or photograph the question and receive a complete solution aligned with CBSE marking schemes.

Is a diagram mandatory for the 5-mark question on wastewater treatment?+

Yes, the CBSE marking scheme typically allocates 2-3 marks for the diagram and 2-3 marks for the explanation in a 5-mark question like 'Describe the steps in wastewater treatment with a flowchart.' The flowchart must be neatly drawn, labeled clearly, and include all treatment stages: bar screens, grit tank, primary clarifier, aeration tank, secondary clarifier, and digester.

What is vermi-processing, and where is it mentioned in Chapter 13?+

Vermi-processing is the use of earthworms to further decompose dried sludge from sewage treatment, converting it into high-quality compost rich in nutrients. It is briefly mentioned in CBSE Class 7 Science Chapter 13 Wastewater Story as an advanced method some treatment plants use to enhance manure quality. Students should know the term but it is rarely a focal exam question.

How can parents ensure their child understands sewage treatment diagrams properly?+

Parents should encourage children to draw and label diagrams repeatedly—practice makes perfect. Use the NCERT textbook diagram as a reference, then draw from memory and compare. CBSETUTOR.ai allows students to upload hand-drawn diagrams via photo; the AI tutor checks labels and provides instant feedback on accuracy and completeness, ensuring exam-ready diagrams every time.

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