Waste: Streams, Silver, and the Drain You Must Not Use

You decide how much waste you will produce, and of what kind, at the moment you choose a process, a dilution and a tray size — which is before you have mixed anything, and long before you are standing over a sink at eleven at night with a jug in your hand wondering what to do with it. That is the argument of this page: waste is a design decision, and the sink is where a decision you did not make gets made for you.

By the end you should be able to name the streams your own darkroom will produce, say what is in each, keep them apart for reasons you can state, label and store them, explain why spent fixer is both the most troublesome and the most valuable thing you will pour out, and — the part no course can do for you — find out what your own local authority actually wants you to do with it.

The streams a silver darkroom produces

Five, and they are chemically incompatible with one another in ways that matter.

Stream	What is in it	Why it is its own stream
Spent developer	alkaline; developing agents, partly oxidised; sulfite; carbonate or hydroxide; bromide released from the film	high pH, organic reducing agents, and an oxygen demand
Spent stop bath	dilute acetic or citric acid, plus carried-over developer	acidic; it is the stream that must never meet the next two
Spent fixer	thiosulfate, in quantity; dissolved silver; sulfite; halide released from the emulsion	silver, and a large oxygen demand; the one worth recovering
Wash water	very dilute everything, mostly thiosulfate	huge volume, low concentration; the stream people forget is a stream
Solids	packaging, contaminated wipes and gloves, film and paper offcuts, spent recovery media	silver-bearing in some cases; not liquid, so a different route

Kodak’s guidance for amateur photographers describes the first four together as photographic effluent and identifies its two key characteristics: pH and the concentration of silver. Developer is alkaline, fixer is acidic, and Kodak notes that when the two are combined as total process effluent the mixture is neutralised. Hold that thought until the section on neutralisation below, because it is true and it is not permission.

One processing session, and where every vessel ends up

1. 1Develop → developer waste — alkaline, organic, oxygen-demanding
2. 2Stop → acid waste, on its own — never combined with fixer or developer waste
3. 3Fix → silver-bearing fixer waste — the valuable one; thiosulfate and dissolved silver
4. 4First wash → the fixer container — the first change carries most of the thiosulfate and silver out of the tank
5. 5Later washes → ask locally — very dilute; the one stream whose route genuinely depends on where you live
6. 6Solids → their own bag or tub — wipes, gloves, absorbent, film and paper offcuts
7. 7The sink — present in the diagram only as the thing nothing points to

Four containers, one bag, and a question. The question is not evasion: the answer differs between countries, between local authorities and between water companies, and it changes.

Why fixer is the different one

What is actually in it

Fixing works by turning an insoluble silver halide into a soluble complex, so a spent fixer is, by design, a solution of dissolved silver.

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing: the silver leaves the emulsion as a thiosulfate complex

Kodak’s guidance for amateurs names the species in the effluent as silver thiosulfate. The chemistry of that complex, its formation constant and why two thiosulfates rather than one is Part XI’s subject; what belongs here is the consequence. Every gram of silver that used to be in your film is now in your fixer bottle, and it is not going to fall out on its own.

How much silver

Kodak’s silver-recovery publication tabulates it. For a black-and-white fixer, the silver concentration of the overflow solution is 3,000 to 7,000 mg/L — three to seven grams of silver in every litre. For comparison, its disposal publication lists the effluent parameters municipalities most often regulate, and gives the mean sewer limit for silver as 1.2 mg/L.

The ratio between those two figures is of order a thousand, and it is the whole argument. That is also why the silver is worth recovering rather than discarding: three to seven grams per litre is not a trace contaminant, it is an ore.

The load on a treatment works

Silver is not the only problem. Kodak’s disposal publication lists the other regulated parameters and their mean limits: biochemical oxygen demand 350 mg/L, chemical oxygen demand 650 mg/L, total suspended solids 200 mg/L, chlorine demand 25 mg/L, pH 5.6 to 9.4, iron 17 mg/L. It explains why biochemical oxygen demand matters, and the explanation is the one worth carrying: micro-organisms in water consume certain chemicals and use up the dissolved oxygen that fish and other aquatic life depend on. A solution rich in reducing agents — which is what a developer and a spent fixer both are — consumes oxygen as it is broken down.

!Two published positions, both from tier-one sources, that do not agree

ILFORD, writing for domestic users in the United Kingdom, says local authorities should have arrangements for receiving small quantities of chemical waste at household waste and recycling centres, usually in a “chemical cupboard”; that chemicals should be bottled separately and appropriately labelled; and that where such routes are not available, small amounts of used chemicals should be flushed down the drain with plenty of water, without mixing them. It advises against septic tanks, and tells trade users to go through a licensed waste operator, noting that recovery of silver from used fixer, film and paper is a key part of the waste treatment process.

Kodak, writing for amateurs in the United States in 1999, says that as a domestic user you are excluded from the pretreatment regulations that bind businesses, and that most photographic chemicals can go to the sewer — developers, stop baths, fixers after silver recovery, and wash waters — while selenium toners and solvents may not. It does not recommend septic systems for photographic waste at all, and it recommends recovering silver before discharge whatever route you use.

This course takes the stricter reading of the two, and states why rather than asserting it. Both sources put silver recovery before discharge. Both are written about jurisdictions with particular rules at particular dates. Neither is your local authority. The course’s rule is therefore: collect every stream separately, take the silver-bearing one to a collection route, and find out what your own authority says about the rest — which is the ILFORD position with its fallback used last rather than first.

The regulatory reality

Waste law is jurisdictional and it changes. This course can tell you the chemistry, which does not change, and the shape of the rules, which does. It cannot tell you what is lawful where you live, and any page that claims to is either out of date or writing about somewhere else.

Here is the shape, with dates, so you know what to go and check.

In England and Wales, the Environment Agency’s waste classification guidance is explicit that nearly all household, commercial and industrial wastes need to be classified, including waste from domestic households. Chapter 09 of the List of Waste is “wastes from the photographic industry”, and it is worth reading because it tells you how a regulator sees your bottles:

Code	Description	Entry type
09 01 01*	water-based developer and activator solutions	absolute hazardous
09 01 04*	fixer solutions	absolute hazardous
09 01 05*	bleach solutions and bleach fixer solutions	absolute hazardous
09 01 06*	wastes containing silver from on-site treatment of photographic wastes	mirror hazardous
09 01 07	photographic film and paper containing silver or silver compounds	absolute non-hazardous
09 01 13*	aqueous liquid waste from on-site reclamation of silver	absolute hazardous

An absolute hazardous entry is one you must use: the waste is hazardous, and no assessment can make it otherwise. So spent developer and spent fixer are, in that framework, hazardous waste — and the guidance adds a note that all aqueous liquid waste from on-site reclamation of silver is hazardous under one of two codes, which is a sentence to remember before Part XII. Film and paper carrying silver, by contrast, is an absolute non-hazardous entry.

The practical household route is separate from all of that: GOV.UK maintains a “find a local hazardous waste disposal service” page, which asks for a postcode and points at your council’s service — and which states on its face that it is available in England and Wales only. Two of the four nations of one country, on one government page, is the clearest possible illustration of why this course does not publish a disposal instruction.

Trade effluent is a separate legal idea and worth knowing exists. Section 118 of the Water Industry Act 1991 provides that the occupier of trade premises may discharge trade effluent into a sewerage undertaker’s public sewers with that undertaker’s consent, and makes it an offence to do so without one. The regime is addressed to trade premises; this course has not read the Act’s definition of that term and does not tell you how it applies to you. What it does tell you is why the section is worth knowing about at all: the domestic question is normally answered by your council’s waste service rather than by your water company, and the moment you begin selling prints you are in a different conversation with a different body.

Elsewhere, in outline and dated. In the United States, the federal effluent guidelines for the photographic point source category, 40 CFR Part 459, apply to point-source discharges from photographic processing and expressly exclude facilities processing 150 square metres (1,600 square feet) per day or less — which is every darkroom this course describes. Where they do apply, they limit silver to 0.14 kg per 1,000 m² of product on any one day. The EPA’s household-hazardous-waste guidance takes the domestic case in general terms: leftover household products that are corrosive or toxic are household hazardous waste, improper disposal includes pouring them down the drain or into storm sewers, and the reader is told to contact their local environmental, health or solid waste agency. Kodak’s own publication adds the useful practical fact that household hazardous waste collection is available to people generating less than 100 kg a month, which is every reader of this course by a wide margin.

📓The one exercise on this page that this course cannot do for you

Find your local authority’s published guidance on household chemical waste. Write in your notebook: the authority’s name, the URL, the date you looked, whether photographic chemicals are named, whether there is a container or quantity limit, and whether you must book. Then repeat it once a year. A dated entry is the difference between “I checked” and “I remember checking”.

Do the same for the two questions this page cannot answer generally: what your authority says about wash water — the very dilute stream — and whether it accepts silver-bearing fixer or expects it to go somewhere else. Acceptance varies between authorities, and this course states that it varies rather than guessing which way yours goes.

Collecting it at home

A liquid you are holding: which stream, which container, which question

1. 11. What is it, and is it silver-bearing?Fixer, bleach-fix, the first wash after fixing, and anything that has touched silver nitrate are silver-bearing. Everything else is not.
2. 22. Is it acidic?Stop bath and anything acid goes to the acid container, alone. Never into fixer or developer waste, where the acid liberates sulfur dioxide from sulfite and thiosulfate.
3. 33. Into the container for its own streamOne container per stream, already labelled, standing in a tray, filled through a funnel, with headspace left.
4. 44. Close it, and write the lineCap it, top up the date-and-volume line on the label, and note it in the waste log.
5. 55. When it is full: the route you checkedThe collection route you looked up and dated — for silver-bearing waste, the one that leads towards recovery.
6. 66. If you do not know: stop and askA sealed, labelled container standing in a tray is a solved problem for weeks. A guess poured away is not recoverable.

Step 2 comes before step 3 because it is the only step whose consequence is immediate, in the room, in the next ten seconds.

One container per stream. Not one big one. The streams are chemically incompatible and they have different destinations, and a mixed container has the properties of the worst of its contents and the value of none of them.

Containers, and the rules that go on them. Rigid, chemically compatible, with a cap that seals; the storage page argues the material question. Leave headspace — a full container has nowhere to put the gas from a slow reaction, and no room for the next session. Stand every container in a tray, as secondary containment. Never fill a bottle that once held food or drink, and never one whose original label is still on it: an unlabelled or wrongly labelled waste container is the single most dangerous object a home laboratory produces, because the next person to pick it up has no way to know.

The label, using the same scheme as everything else you fill:

A waste label, with every field a waste container needs

1. 1Contents, in words — spent fixer — ammonium thiosulfate — SILVER-BEARING
2. 2Source process and dilution — B&W film, rapid fixer 1+4
3. 3Date opened, date closed, running volume — the container is also the log
4. 4Hazard wording from the product safety data sheet — H400, H410 — very toxic to aquatic life with long lasting effects
5. 5Destination, and the date you checked it — so the label carries its own provenance
6. 6Your initials and a telephone number — for whoever handles it after you

An unlabelled container is not waste awaiting disposal; it is an unknown liquid, and nobody can lawfully or safely take it.

Where they stand and for how long. Low, in a tray, out of sunlight, away from food and out of reach of children and animals — the same shelf rules as anything else, because a waste container is a chemical container. How long is a question of the container rather than the contents: a sealed, labelled bottle standing in a tray is stable for as long as it takes you to fill it and get to a collection point. The failure mode is not the chemistry; it is a row of unlabelled bottles nobody can identify.

What must never happen

Four things, each with the reaction or the consequence that makes it a rule rather than a preference.

1. Acid waste tipped into fixer or developer waste. Acid liberates sulfur dioxide from sulfite and from thiosulfate, and it does so at once, in a room, over a container you are leaning over. Thiosulfate meeting an acid also throws down sulfur, which is why a spent fixer that has met a stop bath goes milky as well as smelling. The storage page sets out that chemistry and the equation for the sulfite case; this course writes no equation for the thiosulfate case, because it has not read one in a source that meets its standard.

Kodak’s environmental module for motion-picture processing gives the laboratory version of the rule: acid cleaning solution is collected separately from developer and other processing solutions, because mixing them can emit sulfur dioxide gas. The storage page develops the chemistry; the reason it reappears here is that waste is where people relax, and the acid container is the one that gets used as a general receptacle at the end of a tiring session.

2. Silver-bearing fixer to the drain. Three to seven grams of silver per litre against a mean regulated limit of 1.2 mg/L, an aquatic classification of H400 and H410 for silver compounds, and a recoverable metal thrown away.

3. Anything photographic into a septic system. Both manufacturers say so independently. ILFORD calls it inadvisable because it may compromise the effectiveness of the tank; Kodak states that septic systems do not have the ability to properly treat photographic processing solution waste, that they work by anaerobic biological action, and that it does not recommend them. A septic tank is a small biological reactor, and the streams above are rich in things that kill or overload bacteria.

4. Letting a residue dry out to make it easier to move. It is not easier, and three things go wrong. A dried residue is a dust, so a liquid hazard becomes an inhalation one. A dried silver residue is a solid oxidiser in whatever it dried onto — the silver nitrate page makes that case. And a dried, unidentifiable solid is far harder for a waste service to accept than a labelled bottle of liquid.

Neutralisation, and what it does not do

Neutralisation has a real, narrow use and a large mythology around it.

What it legitimately achieves. It brings the pH of a dilute acid or alkaline rinse into a range that is compatible with drains and with a treatment works. Kodak’s guidance for amateurs recommends neutralisation for its indicator stop bath before discharge, and describes the operation: sodium bicarbonate solution added slowly, because the mixture foams, in a well ventilated area, wearing gloves, goggles and an apron.

What it does nothing for. It does not remove silver. It does not remove thiosulfate. It does not reduce biochemical oxygen demand. It does not change the mass of anything you are about to release; it changes one property, pH, and leaves every other property where it was. Neutralising a spent fixer gives you a pH-neutral spent fixer with all its silver still in it.

✕Dilution is not a disposal method, in three sentences

Adding water to a waste changes its concentration and not its quantity. The same mass of silver, of thiosulfate and of oxygen demand arrives at the same treatment works and the same watercourse, on a slower schedule and in a larger volume. A limit expressed as milligrams per litre is a limit on what a treatment works can handle in the water it receives, not a target you are invited to reach by adding your own water — and the moment you notice that the diluting water also has to go down the same drain, the argument collapses on its own.

There is a real case where combination is not dilution, and it is worth stating precisely because it is the one that sounds like a loophole. Kodak observes that when developer and fixer are combined as total process effluent the mixture is neutralised, giving better compatibility with drain pipes and with treatment at a publicly owned treatment works. That is a genuine chemical effect. It is also a statement about a whole facility’s effluent, made in a document whose central recommendation is to recover the silver first. It is not an argument for pouring your fixer into your developer bottle, which would combine a silver-bearing stream with one that is not and make both harder to route.

Reducing waste at source

The cheapest waste to deal with is the litre you never mixed.

• One-shot against reuse. ILFORD states both sides for its own liquid concentrate: for the highest image quality LC29 should be used one-shot, but for greater economy it can be reused, and 1 litre at 1+9 will process ten films or at 1+19 five — with development times increased by about 10 per cent for each successive film, and a tally kept. That is the trade in full: one-shot buys consistency and costs waste volume; reuse buys economy and costs a bookkeeping obligation that you will forget.
• Dilution choices. A higher dilution uses less concentrate for the same tank, and the concentration page shows what it costs you in measuring precision. Choose the dilution deliberately; it appears in your waste log either way.
• Tank and tray sizes. Mix what the tank holds, not a round number. A 300 ml tank does not need a litre made up.
• Wash economy. ILFORD publishes a fill-and-invert method for films that replaces a running wash: fill, invert five times, drain; refill, invert ten times, drain; refill, invert twenty times, drain; then a final rinse with wetting agent. Three tank-fulls instead of a running tap is an enormous reduction in the largest stream by volume, and it is the manufacturer’s own method rather than a folk economy.
• Buy what you will use. Kodak’s advice to amateurs is exactly this: buy only the supplies you need, and mix only the amount you need, so that you are not disposing of chemistry that went out of date on the shelf. Stock that ages into waste is the most expensive waste there is, because you paid for it twice.

The waste log

One page in the notebook, one line per container.

Date opened	Stream	Container	Date closed	Volume	Route taken	Date checked
2026-09-04	spent fixer, silver-bearing	2 L HDPE, amber

A year of that tells you something no general page can: how much of each stream your own practice produces. That number decides whether silver recovery at home is worth the equipment, which is the question Part XII opens with; it decides what size containers to buy; and it makes the planner’s figures yours rather than somebody else’s.

=What a year is likely to tell you

This course has not yet published its own measured figures, and does not invent them. What it can tell you is the shape of the answer, from the sources on this page. Wash water will dominate by volume — Kodak’s disposal publication says so in as many words, that wash water makes up the largest part of photographic-processing effluent, and uses it to estimate the total. Spent fixer will be small by volume and will carry nearly all the silver, because that is where the silver goes. Solids will be almost entirely packaging. If your own log says something different, that is a result about your practice and worth writing down — most of this course’s numbers will eventually come from readers’ logs rather than from manufacturers’ averages.

What is deferred, and to where

• Silver recovery chemistry — electrolysis, metallic replacement and chemical precipitation, with the recovery efficiencies Kodak states as more than 90, more than 95 and more than 99 per cent, and the small-scale experiment — is Part XII. This page’s job is to make sure the silver still exists when you get there.
• Washing itself, and how little water is actually needed to reach an archival standard, is Part XII as well.
• The chemistry of fixing — the thiosulfate complexes, exhaustion, capacity — is Part XI.
• The standard operating procedures for waste and silver waste live in the SOP library — routing general chemical waste and routing silver-bearing waste — built out of the labelling scheme and the log on this page.

And the sentence that belongs on every page of this course that produces a waste stream, including this one: the chemistry here is general, the law is local, it changes, and you must check your local regulations.

Recap

• Waste is chosen when you choose a process, a dilution and a tray size. By the time you are holding the jug the decision has been made.
• Five streams: spent developer, spent stop, spent fixer, wash water, solids. They are kept apart because they are chemically incompatible and have different destinations.
• Spent black-and-white fixer carries 3,000 to 7,000 mg of silver per litre against a mean regulated sewer limit of 1.2 mg/L. It is simultaneously the most troublesome stream and the only valuable one.
• In England and Wales, spent developer and spent fixer sit under absolute hazardous entries in the List of Waste, film and paper carrying silver under an absolute non-hazardous one, and the household route is your council’s hazardous waste service.
• ILFORD and Kodak give different domestic advice for different jurisdictions at different dates, and both put silver recovery before discharge. This course collects everything and takes the stricter reading.
• Never combine acid waste with fixer or developer waste: sulfur dioxide. Never a septic system. Never let a residue dry out.
• Neutralisation fixes pH and nothing else. Dilution changes concentration, not quantity, and is not a disposal method.
• One labelled container per stream, headspace, in a tray, and a log line for each. The two label fields people omit — the running volume and the date you checked the route — are the two that turn a container into a record.

Check your understanding

Question 1. Explain in one answer why diluting spent fixer heavily is not a disposal method.

• Because dilution takes too much water to be practical
• Because dilution reduces the concentration but not the quantity: the same mass of silver, thiosulfate and oxygen demand still arrives at the treatment works, and the diluting water goes down the same drain
• Because diluted fixer becomes more toxic as the complex breaks down
• Because the silver precipitates out and blocks the trap

Show the answer and why

Answer: Because dilution reduces the concentration but not the quantity: the same mass of silver, thiosulfate and oxygen demand still arrives at the treatment works, and the diluting water goes down the same drain

A regulated limit expressed in milligrams per litre is a limit on what the receiving system can handle in the water it gets, not a target to be met by adding water of your own. The mass released is unchanged, and the diluting water joins the same sewer, so the concentration reaching the works is barely affected anyway. There is a second reason that matters just as much: at 3,000 to 7,000 mg of silver per litre, spent fixer is a recoverable resource, and diluting it makes recovery harder as well as discharge no better.

Question 2. A student combines their spent stop bath and their spent fixer in one container to save space. Name the gas that may be produced and the control that prevents it.

• Chlorine; the control is ventilation
• Sulfur dioxide, from acid acting on thiosulfate and sulfite; the control is one container per stream, with the acid stream never combined with either the fixer or the developer stream
• Hydrogen sulfide; the control is keeping the container cold
• Ammonia; the control is a sealed cap

Show the answer and why

Answer: Sulfur dioxide, from acid acting on thiosulfate and sulfite; the control is one container per stream, with the acid stream never combined with either the fixer or the developer stream

Acid liberates sulfur dioxide from thiosulfate and from sulfite, and the thiosulfate reaction also throws down a milky precipitate of sulfur, so the container gives you a gas and a sludge at once. Kodak's own environmental guidance for processing laboratories gives the rule in exactly this form: acid cleaning solution is collected separately because mixing it with developer and other processing solutions can emit sulfur dioxide. Ventilation is a mitigation, not a control, and it is the wrong answer here because the hazard is designed out for free by keeping the streams apart. Chlorine belongs to acid on hypochlorite bleach, which is the other domestic version of the same mistake.

Question 3. In England and Wales, how does the Environment Agency's waste classification guidance treat spent photographic fixer and spent developer?

• As household waste, because they come from a home
• As absolute hazardous entries in the List of Waste - 09 01 04* for fixer solutions and 09 01 01* for water-based developer solutions - which means the code must be used and no assessment can make the waste non-hazardous
• As mirror entries, hazardous only if a test shows they exceed a threshold
• They are not classified at all below a volume threshold

Show the answer and why

Answer: As absolute hazardous entries in the List of Waste - 09 01 04* for fixer solutions and 09 01 01* for water-based developer solutions - which means the code must be used and no assessment can make the waste non-hazardous

Chapter 09 of the List of Waste covers wastes from the photographic industry, and both fixer solutions and water-based developer solutions carry an asterisk and an absolute hazardous entry type. An absolute entry is one that must be used; a mirror entry is the kind that does require an assessment, and 09 01 06*, wastes containing silver from on-site treatment, is one of those. The guidance also states that nearly all wastes need classifying, including waste from domestic households. What this does not settle is the practical route for a householder, which is a separate question answered by your local authority's hazardous waste service - and that is why the classification and the route are looked up separately.

Question 4. Kodak observes that combining developer and fixer as total process effluent neutralises the mixture. Why is that not an argument for pouring your fixer into your developer waste container?

• Because the two would react violently
• Because it is a statement about a facility's overall effluent from a document that recommends recovering the silver first, and combining them puts silver into a stream that had none, making both harder to route
• Because developer waste is acidic and fixer is alkaline, so they would not neutralise
• Because neutral pH is worse for a treatment works than an alkaline one

Show the answer and why

Answer: Because it is a statement about a facility's overall effluent from a document that recommends recovering the silver first, and combining them puts silver into a stream that had none, making both harder to route

The chemical observation is true - developer is alkaline, fixer is acidic, and mixing them moves the pH towards neutral, which is easier on drain pipes and on a treatment works. The mistake is scope. Kodak is describing what happens to the combined output of a facility that has already recovered its silver, not recommending that you merge two collection containers. Merging them contaminates a silver-free stream with silver, destroys the value of the fixer, and leaves you with one container that has the disposal constraints of both. Note also what neutralisation does not touch: silver, thiosulfate and oxygen demand are all exactly where they were.

Question 5. Which single change most reduces the volume of waste a film-developing session produces?

• Using a higher dilution of developer
• Replacing a running wash with ILFORD's fill-and-invert method: three tank-fulls of water instead of a tap left running
• Reusing the developer for a second film
• Mixing a litre of developer at a time instead of 300 ml

Show the answer and why

Answer: Replacing a running wash with ILFORD's fill-and-invert method: three tank-fulls of water instead of a tap left running

Wash water dominates the volume of everything a darkroom produces, by a long way, so the largest saving is there. ILFORD publishes the method: fill the tank, invert five times, drain; refill, invert ten times, drain; refill, invert twenty times, drain; then a final rinse with wetting agent. The other options are real economies of different kinds - higher dilution saves concentrate, reuse saves both at the cost of a development-time tally and some consistency - but neither touches the biggest stream. The last option goes the wrong way: mixing more than the tank holds converts money directly into waste.

Question 6. Why does this course insist that a waste container carries the date on which you checked its disposal route, as well as the route itself?

• Because waste containers legally expire
• Because disposal rules are jurisdictional and change, so a route recorded without a date is a claim you can no longer evaluate - and the container is also the record
• Because the chemistry of the contents changes over time
• Because a dated label proves the waste is yours

Show the answer and why

Answer: Because disposal rules are jurisdictional and change, so a route recorded without a date is a claim you can no longer evaluate - and the container is also the record

The whole argument of this page is that the chemistry is general and the law is local and moving. A route written on a label with no date might have been checked last week or three years ago, and there is no way to tell which - so it cannot be relied on and will have to be checked again. With a date it is either current or visibly stale, and either way it is information. The same logic runs through this course: a safety data sheet is recorded with its version and the date you read it, a calibration check with the date it was made, and a local waste rule with the date you looked it up.

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