Can I use a storm-water pond as a back-up source of water?

Storm-water ponds are the closest alternative source of water for many people living in urban areas. In an emergency this source of water may be all that is available to you.  Eventually any water you have stored will be consumed and the water in a storm-water pond may be the difference between life and death.  With the proper treatment your local storm-water pond can be a great backup source of drinking water.

Industrial storm-water pond (source:

Storm-water presents a unique set of challenges during treatment. Because storm-water ponds collect surface water, the water is exposed to all the contaminants on the ground in the catchment area. This includes but is not limited to pesticides and fertilizers applied to lawns, motor oil and gasoline leaking from vehicles and litter like cigarette buts. It all ends up is the storm-water pond. Those chemicals are already in storm-water ponds on a normal day. During an emergency there may be additional contamination from sewage runoff from an overloaded or broken sewage system. The water in the pond will also contain all the microorganisms like ecoli, giardia and cryptosporidium normally in surface water. Any one of these will make you very sick if you get infected with them.
Finally, there will be high levels of nitrates in storm-water ponds. Too much nitrates consumed by young children can cause blue baby syndrome.

The first step in treating water from a storm-water pond is straining. Straining the water through a cloth or loose sand filter will remove large particles (ones you could pick up with your fingers). Remove as much of the suspended particles from the water as you can. Straining the water first will extend the life of your proper water filter.

If you have a clarifying agent like aluminum sulfate, this is the best time to add it to the water.  It will make contaminants too small to be filtered become attracted to each other and become significantly larger.  Larger particles are easier to remove from the water. Let the water sit still for at least 30 minutes without disturbing it.  All the newly formed large particles (called floc) will sink to the bottom.  When you take the water from this container, make sure you leave the majority of the settled material at the bottom of the container.

Urban Storm-water pond (source:

The next step is to filter the water. Filter the water even if it looks clear, the human eye is five times too weak to detect dangerous levels of particles. Filter the water at least once through an activated carbon filter. Activated carbon is known to remove many different chemicals from water including pesticides, chlorine and fluoride. Activated carbon is not the same as charcoal. Charcoal is similar, it can remove toxins from water but it is nowhere near the efficiency of activated carbon.

The third step is oxidation. Oxidation will help with disinfection as most disinfectant chemicals are also oxidizers. Chemicals like sodium hypochlorite and potassium permanganate are both oxidizers and disinfectants. Oxidation will break down many of the remaining contaminants and inactivate many of the remaining bacteria. Keep adding the oxidizer/disinfectant till you can detect a residual after 20 minutes. The 20 minutes is the minimum you should wait for a gallon of water. Wait longer for larger volumes. This is because oxidation is a chemical reaction that isn’t instant. It needs time to complete the reaction.

The fourth step is to filter the water again. Filtering again is necessary because the disinfection/oxidation step will create some potentially carcinogenic byproducts. We filter before oxidation to minimize the amount of chlorine (or other chemical) and to limit the possibility of forming dangerous byproducts. We filter the second time to remove any byproducts that have been formed.

The final step is to boil the water.  This will help with disinfection, but the main goal of boiling at this point is to remove any volatile chemicals.  Any chemical with a boiling point lower than water will be removed after boiling.

A note about disinfection.  If all of these steps are followed there is no need for a step dedicated for disinfection.  Between the oxidation and the boiling of the water any microorganisms will be inactivated.  If you are storing the water for a long time then add some sodium hypochlorite for a residual disinfectant.  The residual disinfectant will prevent the water from becoming recontaminated before you drink it.

One additional possible step is to aerate the water.  Ponds are frequently stagnant.  Stagnant water is green with algae, it smells bad and tastes worse. After the water is made potable, transfer the water back and forth between two glasses. This adds oxygen to the water and will make the water taste better.

This may seem like a lot of work for something as small as a storm-water pond.  What I have described are the basic steps to turn the potentially toxic water in the pond into clean and safe drinking water.

Terrorist Attack and Water Systems

Water systems are distributed networks of pipes, pumps and reservoirs. Like all distributed networks they can be very difficult to protect from vandalism and terrorist attack. There are two broad types of attacks that could hit a water system. The first type is an attack on the quantity of water available (physical supply) and the second is an attack on the quality of water. The end result of both types of attack is a lack of potable water entering your home.

The greatest defense for a water system is that most of it is underground. It is very difficult to access most parts of water distribution systems. Even for the operators of the system it is time consuming and disruptive to the wider community. If anyone unauthorized to dig in a road to access a watermain they will be reported to the authorities in the form of complaints about traffic or noise.
I think it goes without saying that depending on public complaints to defend against terrorism is nowhere near secure.

Water Treatment Plant (source:

The exposed parts of water systems are water treatment facilities, reservoirs and fire hydrants. These are the points where the system is at the surface and easily accessible. These different points also offer different security concerns.

Treatment facilities are as secure as any factory or industrial facility will be. The treatment facility I work at is always locked and there are a limited number keys. Then there is an electronic alarm system which brings a human on site if there is an intrusion alarm. There is also a human dispatched if the communication link is broken. A large city water treatment facility will most likely be manned twenty-four hours a day. The biggest weakness here is that properly armed people can force their way in and destroy the building if they so desire. Or they can contaminate the reservoir on site (if they know how).

Reservoirs of treated water are next most likely place for a terrorist attack. The biggest weakness here is that reservoirs are almost never manned during the day. They will be visited most days, but rarely will people be there all day. The good news is that water in reservoirs is monitored constantly(usually) with automatic analyzers. Reservoirs can be destroyed, and the water within wasted. Or the quality of the water can be destroyed this is where chemicals could be added easily.

The remaining pieces of the system are fire hydrants. Fire hydrants pose a unique risk to water security. It is very easy to add chemicals to a fire hydrant. It is however not easy to get that chemical into the water supply. This is because of the construction of the hydrant itself and the pressure in the system. There is a valve at the bottom of the hydrant which isolates the water. In order to add chemicals to a fire hydrant you also have to lower the system pressure which is rarely easy to do undetected. At this point I want you to remember the scene in Batman Begins where Sandman is dumping his psychotropic drug into a cracked watermain. I don’t expect realistic depictions from the movie, I do however want you to know this is nowhere near realistic. Watermains are pressurized, when they crack water shoots out at anywhere from 50 to 100psi. This is enough to erode foundations of buildings and the all soil around the break creating massive sinkholes. It is not something you can pour chemicals into. This is what it looks like when watermains break.

Just adding chemicals to water, is not as effective as it appears on the surface.  Most water supplies contain residual disinfectants, usually chlorine.  Disinfectants are highly reactive chemicals, they aren’t limited to just killing bacteria.  Highly reactive chemicals often react with other chemicals.  I am obviously oversimplifying the chemistry involved, but it is true that a large portion of any chemical added to water will be consumed by the chlorine in the water.

Adding Chemicals to Water (source:

Cyber attack is another way water systems are vulnerable.  You may wonder why water treatment facilities are connected to the internet and the answer is for remote monitoring and control.  It may seem like an unwarranted risk having these facilities connected to the web.  It is not an unwarranted risk at all.  The likelihood of the automated system needing an intervention that cannot wait for someone to be onsite is greater than a targeted cyber attack. Keep in mind that even normal breakdowns of the watersystem can cause illness and even death.  These need to be responded to and are just as important as preventing cyber attack. I am not a technology expert so I will leave it to other people to suggest the best firewall setup.   Another thing to note about cyber attack on a water treatment facility is that even if the attacker is successful and shuts down the control computer, the facility can still be controlled manually.

Terrorism is something that needs to be addressed when it comes to water systems.  People inside and outside the system need to be aware of the risks and what can be done for protecting the security of out water.

Water Preparedness: Common Beginner Mistakes

Are you thinking about starting to store water? How about emergency water treatment? Getting started can be a very daunting task. Where should efforts be focused? What pitfalls should be avoided?  This article will explore a few of the mistakes I see people make when they start to take their personal water security seriously

Don’t be left without potable water. Avoid beginner mistakes. (source: always

The very first thing to learn is that there is no magic bullet. There is never a single product or technique which will always make water safe to drink. Combining, knowledge, multiple storage/treatment techniques and multiple products for storage/treatment is the best way to guarantee a safe source of drinking water for yourself and your family.  This logic or philosophy of combining as many protections as possible is used by municipal water systems all across North America.  It is referred to as a multi-barrier approach and it boils down to having many different protective measures to prevent contamination, in the event that one barrier fails, there are still many others in place.  To put it in layman’s terms, when it comes to water security, it isn’t a good idea to put all your eggs in one basket.

Marketing campaigns will make all sorts of claims about water products. Some will be irrelevant, like claims of BPA free plastic when the product is made from a type of plastic that never had BPA. Other claims will be over stated. The claim that is most often overstated is the number of times a water treatment product can be used. The quality of the water being treated is too variable for any company to give you an absolute number of times. This might not be done to deceive you. It could just be that the water they tested the product with was easier to treat then your water. Remember that no product will make the water perfect.  They will make the water safer when used correctly, if used incorrectly many water treatment products can make the water significantly more dangerous to drink.

Water needs to be stored in an appropriate container. This container needs to be able to physically hold the weight of the water and not leach any chemicals into the water. Assuming any garbage can sized container is appropriate will at best lead to soggy disappointment and at worst a severe case of gastrointestinal disease. For more information on water storage, read The Why? How? and How Much? of Water Storage?

Once your water is stored it needs to be kept safe. Water can become contaminated at any time. Anytime the container is open there is potential for contamination to occur. Read this article to find out what to do when your backup source of water becomes contaminated. The assumption that water only needs to be treated once is false. What was once safe to drink may be very dangerous when you need it if your aren’t protective of your supply. Water can turn stagnant when stored for long periods of time. Stagnation while not a health hazard is a taste hazard. Stagnant water tastes bad. Adding air to the water is how you relieve stagnation. Adding air is as simple as passing the water from one glass to another repeatedly or stirring the reservoir. The goal is to increase surface contact between the atmosphere and the water. It is important that aeration of the water will also remove the remaining chlorine (if any was present) in the water. If you are aerating the reservoir make sure you add some more disinfectant. Do this so you can keep your disinfectant residual high enough to keep the water contamination free.

Another mistake people make, is they store water but make no changes for reducing the water they use. Forgetting to change behavior during a crisis is probably the biggest mistake beginners make. Different situations require different behavior, this applies to your personal water use. You will be amazed at how much water is used if you aren’t careful. What could last a week might be used in a day and then you will understand the true value of water conservation. This mistake can also happen in more than just your water use.  For more information on water conservation read why water conservation is a prepper’s must do.

The single biggest mistake beginners is they assume that they can learn how to treat water later. Later becomes too late and then it can become fatal. It is very difficult to learn something complicated like water treatment when your life depends on it. Learning as much as you can before an emergency strikes is the single best thing you can do to stay safe.

This article covers just a few of the common mistakes I see people make when they start taking their water security more seriously.  There are many more mistakes that can be made and no one person has the perfect solution to them all. Water security is something that needs to be tailored to each person or family’s needs.  Have you found any common mistakes while preparing for water shortages? If so, leave a comment below, I would love to hear them.

Why?, How? And How Much? Of Storeing Water

Storing water is often the fist step in becoming more prepared. People need water everyday for a variety of reasons. Humans can go up to three days without water, however the effects of dehydration can be felt in as little as a few hours if you aren’t careful. In an emergency water use and water needs will change and they will never change in ways where you need less water.

On the surface storing water is just filling containers with water and walking away. If that is all you do then you will regret walking away relatively quickly. The only thing worse than ignoring your stored water is not having any stored water.

Why store water? It buys you time. Time to “wait it out” or time to find more water. This time will allow you to focus on and accomplish other tasks. Other demands for your time will abound in even a minor emergency. If keeping some water around means you can focus on security or damage control then it is definitely worth the effort.  I have written another article about why a back up supply of water is important.

Plastic Bottles are a Simple Way to Store Water (source:

How water can be stored depends on a few factors. Cost and storage space are the biggest factors affecting how you store water. Purchasing bottled water is a good way to start for small places with small amounts of storage. Bottled water is can be very expensive if you want to store a lot of water (more than 72hrs).

The next option is a larger container designed holding water. These containers are available is almost every size. If you can’t find the size you want in a store, there are companies that can make custom shapes and sizes. There are very few limits to the type of container. I mentioned earlier that the container needs to be designed for water. This is for two reasons. First, not all plastics are created equal. Some plastics can leach harmful chemicals like BPA into the water. The best type of plastic is high density polyethelene (HDPE).  HDPE is used on all sorts of applications, including water/chemical tanks and watermains.  Look for the symbol of a triange made of arrows around the number 2 and the letters HDPE underneath to make sure it is the correct one.

HDPE Symbol (source:

Second, water is heavy. One liter of water weighs one kilogram (2.2 pounds). If the container isn’t designed for water there is a good chance it will break. For example a rain barrel holds around 400L, that barrel full of water will weigh over 400kg (or approximately 881lbs).  Below is a photo of a 1000L container, when full this container weighs one metric tonne (1000kg or 2204lbs).  The surface under this container needs to be secure and able to support the weight.  You also need to factor this weight into the support for the container and the shelving unit you store the water on.

1000L or 275 Gal Tote Excellent for Storing Water (source:

Larger tanks need to be air tight. Otherwise the water remains open to contamination. Read here to learn what to do when your back up source of water becomes contaminated. The access point if it is on the top also needs to be air tight when closed. The access point needs to be raised above the rest of the tank so no water/cleaners/dust flows into the tank. If the water does become contaminated, read What to do when your back up supply is contaminated.  The tank will also need a spigot for accessing the water in the tank. The spigot should be installed just above the bottom of the tank (like it is in the picture above). Keeping the spigot just off the bottom avoids drawing any sediment that has settled on the bottom of the tank. Raising the spigot also makes it easier to fill containers.

There is are many theories for what kind of container is best. One big one or many little ones or a combination of sizes. This is a decision everyone needs to decide for themselves.  The general theory for using small bottles is that you can rotate your supply and if a small container becomes contaminated or leaks then you still have a safe supply in other bottles.  The general theory in support of large containers is that you can store a lot more water. You can also put pumps into large tanks an pump water to other areas.  With a pump and some tubing you can have a back up plumbing system or pump directly into your existing plumbing.

Now for the big question, how much water do you need to store?  According to the USGS, the average American household uses 80 to 100 gallons per person of water a day (302 to 378 L/person/day).  Now this is for ALL uses, many of which are in our appliances like dishwashers and washing machines.  But if you want everything to be EXACTLY as normal, then 100 gallons per person per day is what you need to store.  A family of four, will need 400 gallons/day and most emergencies are longer than one day.  This will quickly become an unreasonable amount of water to store.  This is where water conservation comes in.  Using less water makes your supply last longer.  I can’t give you a minimum storage volume.  The minimum amount of water that a human needs is variable on too many factors, like age (seniors and babies need more), climate, external temperature (hot and cold), altitude, health conditions, illnesses, type of disaster (some require a lot more washing), pregnancy, activity level, existing hydration levels and this list can be a lot longer. I will say this, having more water than you need is better than not having enough.  Listen to what your local authorities are recommending you store for your area and treat that as a minimum.  The correct answer of how much water to store is in between the 100 gal/person/day and the minimum recommended for your area and time of year.

I hope this article helped you understand a little more about the importance of storing water and a lot more on how to get your water supply started.  Everyone’s water storage solution is unique to them.  Balancing space and money mean that you will have to develop a system that works for you.

Water Survival During the Zombie Apocalypse

First off a bit of a disclaimer, I am fully aware that zombies are not real.  I am however highly entertained by zombies and all things undead.  Since the Omega Man Journal is about water and survival, that lead to the obvious thought experiment of what challenges a zombie outbreak would have on our ability to get safe drinking water.

Zombie, Zombies, Undead, Scary Zombie,

Zombie Rage Face (

The first thing to appear after the dead rise will be panic. Fear will be rampant and many people will be operating on their fight or flight instincts others will be holed up at home. This includes the people who work at municipal water treatment plants. Water facilities, although automated still depend heavily on people to operate them. When those people stop going to work or are already zombie chow then the water will stop shortly thereafter. To see what it would look like when a water system gets shut down and roughly how long it would take read Grid Shutdown: How Long Will The Water Last. The same will be true of the sewage systems. Read Grid Shutdown: Why Is There S#!t In My Basement, Sewage Emergency: Thunder Bay Flooding and A City Without Sanitation to see just how disgusting our once clean (or not so clean) cities will become.

Zombie, Horde, Zombie horde, the walking dead, set pictures

Zombie Horde (

Speaking of sanitation, people will die from unsanitary conditions. That means dead bodies, not just zombies, but regular dead bodies. Cholera is an excellent example of a waterborne disease that is a direct result of decomposing animal tissues in a water supply. Thirst will drive people to the nearest supply of water, then many will die on the banks and contaminate the lakes and rivers. Remember at this point I’m not talking about zombie contamination of water sources. That’s coming up later. This is a real danger when thirsty people or animals die in water supplies.

Depending on how zombies are created there may be a serious water vulernability. There are already bacteria, viruses, parasites and other micro-organisms that use both water and human bodies as part of their natural life cycle. Typically we call the effect of these micro-organisms “water borne disease”. Also there are already parasites that can take control of other organisms even to the point of making the host suicidal. Specifically there is a fungus that will radically alter the behaviour of ants turning them into zombie ants. (read more about zombie ants).  We are talking about total control of the host for the benefit of the parasite. Finally there are countless micro-organisms and insects that thrive and depend on rotting flesh to survive either for food or as a vital part of a life cycle stage like maggots turning into flies.

Bacteria thrive on and cause decomposition of deceased organic matter

Those three characteristics which already exist, put a zombie making organism on the edge of being possible. Nature has already made all the organisms necessary to create a zombie, luckily for now the necessary skills are in different organisms and target different organisms. The problem is, if nature caused one organism to eat rotting flesh, it can teach another organism to do the same. Same thing with learning to disperse through water and to take control of other organisms.

Zombie Ant With Fungus Growing Out Of Its Head (

Let’s assume this is the cause of zombification, what then can anyone do to protect themselves? The good news is that modern water treatment is very good at removing and inactivating micro-organisms. The combination of chemically assisted filtration and disinfection should (if done correctly) remove 99.999% of micro-organisms. Depending on the size of this fictitious zombie bug/parasite/virus it might be possible to remove even more than 99.999% if it is on the large end of the size scale. If it is a virus, which is the smallest type of micro-organism then removing 100% of the z-virus will be next to impossible. That means we better hope that there is a disinfection method that can kill the virus either chlorine, UV, ozone or boiling.

Zombies Again (

One thing many people haven’t thought about when they are talking about zombies and water.  Water is very heavy.  If you have to haul water from its source to your secure facility it will become very difficult to run at the same time.  Now vehicles are an option as are hand carts and if you have the resources pumps and pipes are best.  Just remember that they are all noisier and may attract unwanted attention from nearby zombies.

Those are some of the challenges I see affecting our ability to drink safe potable water in the event of a zombie outbreak.  I tried to be as true to real science as is possible when talking about zombies.  At the very least I hope you were entertained.  Can you think of anything I missed?

Septic Tank Alternatives: Lagoons and Constructed Wetlands

Septic tanks and a subterranean discharge is the single largest way people rural communities treat waste water. That is, in places where the soil, topography and hydrology allow for underground discharge.
The other limitation of septic systems is the capacity, if you expect large volumes of water then you need a large tank/multiple tanks and that can be very expensive.

Constructed Wetland at the Edmonton International Airport source:

I have already written a fair amount about septic systems, and in this article I am going to explore some of the other options for onsite watsewater treatment. Then next two most common options are Lagoons and constructed wetlands. A sewage lagoon is a water-tight earthen berm including the bottom. The construction is not complex, but it can become land intensive depending on the size. Constructed wetlands are identical to lagoons but they are allowed to grow over with vegetation forming a complete wetland ecosystem.

Sewage Lagoon Site Plan source:

Lagoons are sometimes referred to as stabilization ponds. In a basic level, they reduce organic contamination in domestic sewage down to levels suitable for release back into the environment with minimal impact. If they are operated correctly, they can have a positive effect on the receiving water way. The lagoons I operated have lower phosphate, ammonia and nitrogen levels than the river they discharge into.

Constructed Wetland Diagram source:

Constructed Wetland Diagram source:

Lagoons are best constructed with a bulldozer or front end loader because it will be easier to create even depth impermeable layers of soil. The impermeable layers are constructed by compacting clay based soils into a water tight layer. If the available soil is impractical for compaction, a layer of sand topped with a plastic/rubber liner at least 30mm thick. There are professional installers and commercial DIY options available.  Below is a table from University of Missouri outlining the size and space a lagoon requires.







1 TO 2

























You may be wondering how a lagoon/wetland system can handle larger volumes than a septic system. The answer is oxygen. Septic systems operate under anaerobic conditions meaning they are oxygen deprived. Lagoons and wetlands are facultative. Facultative environments are partially aerobic, and partially anaerobic. In a lagoon the surface is aerobic as oxygen from the atmosphere is added to the water and the deeper you go the more anaerobic conditions become. Aerobic conditions allow a much more efficient bacteria to break down the waste. These more efficient bacteria are surprisingly named aerobic bacteria.

Aerobic and Anaerobic Processes in a Sewage Lagoon source:

If a lagoon or wetland is operating properly, there will be a musty smell.  This is the same smell produced by natural wetlands, if you have ever been to a swamp you know the smell I am referring to.  If the oxygen balance is disrupted, for any reason, either overloading or ice cover or chemical contamination the process will turn septic/anaerobic.  Then the lagoon will produce a wide bouquet of odors, most notably a rotten egg smell caused by the formation of hydrogen sulfide.  If you have ever opened a septic tank and caught a whiff, that is the smell I am referring to.  If you decide to treat your onsite waste this way, maintenance of the oxygen balance is critical to avoid the inferior treatment and smells of anaerobic conditions.  Simple things like keeping trees at least 50 feet away from the lagoon will add more oxygen to the water by allowing for more sunlinght (photosysthisis produces oxygen) and wind contact (physically transfers oxygen to the water).  In extreme cases pumping water so it circulates to the surface will also add a lot of oxygen, as will pumping air directly into the water via an air compressor and diffuser will almost guarantee you never see anaerobic conditions.

Constructed Wetlands in Series source:

Sometimes waste systems are combined and there is a septic tank that feeds into a lagoon or wetland and then into a receiving water system.  This arrangement can improve all around wastewater treatment and extend the life of the entire system.  If your local regulations and soil conditions allow for these types of waste water treatment they are definitely worth considering.  Properly maintained and properly designed they can become an attractive and functional feature to any property.

Survival Bunkers: What Features Should They Have?

Survival bunkers are coveted by many as the ultimate survival preparation possible, providing it is full of food and water. The common elements for any shelter are food, drinking water, air circulation and waste removal. Honourable mention goes to electrical power and heating and cooling. The extent and the capacity of these essentials depends on the purpose of the shelter or bunker and how long it will be occupied. The characteristic that almost can go without saying, storage, lots of storage.

Many of the features described in this article are possible to build into almost any structure, some will only be possible or necessary in a fortified bunker. One note on fortified structures, many municipalities have made fortified homes illegal. This was done where I live to combat organised crime, and make it possible to evict criminal organisations, easier to raid these structures and to discourage to setting up of a headquarters in the first place. Be aware of what is legal before you start to fortify any structure.

The first decision to make is if your shelter will be underground or above ground. Both options have advantages and disadvantages. Underground bunkers are easily hidden and offer excellent concealment. The ground around the walls also offers protection from most explosions and most forms of natural disasters. Underground is a great place to hide. Underground is not a great place to defend. Civil unrest can easily exploit a weakness in all underground structures, if all the exits are found by an aggressive party then all inhabitants of the bunker are trapped. All anyone has to do is wait with weapons pointed at the doors, because the doors will eventually open when supplies run out. They won’t even have to wait that long. Blocking air vents, adding toxic gasses and flooding the bunker with water will flush people out faster. The primary security of underground bunkers is secrecy. Keep the location and if possible the existence of any bunker secret. This may even be more important than the strength of the walls.
Whether you choose to build above ground or underground there are features that are vital for both.
Multiple exits, are needed. There is little use in getting trapped in your survival shelter just to die later. Always have another way out. Speaking of exits, installing a barrier that forms a ninety degree bend outside the door will inhibit people from knocking the door down. A cinder block or concrete construction with just enough room to open the door will prevent anyone from swinging a battering ram against the door.

Bunkers that expect to be occupied for extended periods of time should have a UV growth lighting system. Something to at least start some seedlings. Volcanic eruption can create shorter growing seasons. Giving your food plants a head start could make all the difference between harvest and starvation. It would be awesome to have an underground farm that can grow enough food for your entire family, but I would bet money that it is too impractical to be a viable option.

Air circulation is also critical. Simple air circulation can occur with two holes and a fan. Easy to install but the can and will leave you open to any airborne contaminants or contagions. Screens and U-bends will keep out rodents and water. HEPA filters remove many small particles and some airborne bacteria and viruses. You can also use UV light to disinfect the air before it is circulated. And it is best done right after the HEPA filter.
Everything else requires a positive pressure system to keep any contaminants outside. Positive pressure is achieved when air is blown into a room faster than it can be released. This means air will constantly push out of any leaking point and physically push away from the bunker. Even when doors are opened the air will push our preventing contaminants from entering.
This is the opposite of an isolation room, like the ones in a hospital. Isolation rooms remove air from a room faster than air can enter the room. This means air is pulled inside constantly and out to an air disinfection system. This keeps the airborne contagion contained in the one room even when doors are opened.
Both positive and negative pressure systems have higher electricity costs and that could limit their applications.

Water needs mentioning. A source of drinking water is essential as well. Length of stay is the master here. If you only have a storm shelter then you can probably get away with storage of water. If you plan on outlasting extended civil unrest, then a well with access inside the bunker is preferred. A method of disinfection of drinking water is also preferential.

Next is waste, all the garbage and sewage needs to be dealt with. A septic tank can receive the sewage easily enough if the terrain allows for it. Garbage can be incinerated from within the bunker. For more on solid waste and liquid waste management see my other articles.

Fortification is a useful way to help survive many situations. Whether you expect a tornado or World War Three, having a place to retreat to that can take a beating could save your life. Anything that could save your life one day is always worth a closer look.