Defence

[Tibbs735]

With regards to water, redundancy is your friend. You want multiple methods to produce safe water. Filtering and boiling is for short term, as is distilling. However these use a lot of energy.
Your best bet, along with being strict with water use and waste, would be a biosand filter. The link is a diagram:
http://practicalaction.org/image/bio-san...l-plan.jpg
]It totally nerfs anything living in the water, removing any chance of death by diarrhea.
It does this by forming an ecosystem on the top layer of sand that eats bacteria, worms, protozoa, viruses and organic material in the water.
The nasties also die due to lack of sunlight, air and food.
Many come to the end of their natural life cycle, unable to multiply.
They also stick to the slightly charged sand particles.
You could set it up with a large rain tub, a bucket with holes for a diffuser, and some flexible tubing to dispense water. By pouring water in to the filter, you are pushing water out of the bottom of the barrel and out of the tube due to pressure. The filter also reduces heavy metals, and other pollutants.
Crucially, it requires no spare filters and little maintenance. To clean it, you remove the diffuse bucket, pour in a jug of clean water and swirl it around in the sand. Let the sand settle, them remove said water. Repeat until the water is clean.
It might be a good idea to run some water through the clean ed filter for a few days without using it, to allow the bio filter to reform. It would be best to have several running at once, adding the filtered water to a reservoir tank. Some people like to add a proportional amount of bleach as insurance.
A video explaining it all:
http://www.youtube.com/watch?v=hb0xf3mRbJM

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Defence:
Threats have to detect you to get to you, its unlikely they will stumble across you by chance. By avoiding large roads, you avoid refugee lines of drift. Maintain discipline with smoke and cooking smells. Heating up some beef stew will bring them running, walking and crawling. Light discipline is crucial, as a lit house will stand out when the power grid is down and the nights are pitch black. For the first few months, whilst there are still large amounts of people around, you have to stay low, or you will be swamped. However as time goes on, only the self sufficient and the brutally skilled raiders will be alive, so security will remain crucial for a long time.
Decide your boundaries now, when will you shoot? When will you show mercy? Decide now to remove hesitation.
You want a large tribe of ideally 20-25 fighters. This allows for a continuous lookout and patrols. Have your immediate action drills rehearsed with your group now, so sudden threats can be dealt with E.g a convoy of 4x4's racing down the road towards your retreat. Imminent threats, like the golden horde refugees, can be dealt via ambushes at pre-determined choke points. Surprise them, search them, dispense charity if its safe to do so. You will probably have to shoot, as you will be on the map forever afterwards.
Don't let people know you have food. Hide it. Be stealthy.
Lay strands of wire taught at random, ankle to knee level heights and directions across the lawn, to stop people just running to your house. A fireproof roof would be a good prior investment against firebombs, if any petrol is left. Chain-link fence over windows would probably cause solid wine bottles to bounce off and land on the ground.
Most people will probably freak-out and piss themselves in the face of a solid defense. Up until then,they will have been preying on the weak.
Take no chances with shooting. Its the 1% alive guys that you turn your back to that kill you. Unless you sever the spine or destroy their brain, they will be screaming on the ground for few minutes.
Body Disposal
Feed bodies to pigs. There will most likely be plenty of wild ones escaping from factory farms.
Alternatively, just bury them. If you want to be really freaky, a live yogurt enema will speed u decay greatly.
Hope that helps!