I’ve brought this collection together based on experience with S.P.A.N.C technicians and despite national norms things vary from technician to technician.  
As you've  no doubt  been surfing to find out information, be aware of certain manufacturers sites that  throw doom on anything that they don't manufacture, installers that claim they do the paperwork...  after installation and other that just plagiarise somebody elses site.
There were changes to the rules for installation back in september 2009  which allowed a far greater choice of systems to be installed, or at least in theory. The manufacturers had to get approval for their systems & this has taken a while to filter through (unintentional pun) but it does now offer more options and there will be more as they become approved.
While offering these options it does not cut the cost, as a rule of thumb the cost is inversely proportionally to the area the system will take up.  Property without much land now has a greater chance to have its own sewage treatment plant but will invariably cost more than a classic epandage system.  

S.P.A.N.C
The rather amusing acronym stands for Service Public Assainissement Non Collectif.  These are the guys that assess your proposed installation and then inspect the system before it is covered.  This also give you the client an impartial verification that our work conforms to the required standards.  We of course arrange all the paperwork and meetings with the technician.

Siting of the system
Wells
Contrary to popular belief a system can be less than 35metres from a well if the well water is not used for human consumption.  Almost all houses are on mains water these days.

Boundaries & Trees
Any system should be 3metres from your boundary and 3 metres from any trees, though as trees grow the further you can keep the system from trees the better. The 3 metres from the boundary is not set in stone. The regulations say “about” If you don’t have that much free space then each installation is decided on its own merits by the S.P.A.N.C technician.

Sizing the system
Size of the septic tank (fosse septique) depends on the size of the house.  It doesn’t matter if it’s a main residence or only a holiday home or that there’s only two of you that live in a six bedroom house.
The size is decided by the number of main rooms. Main rooms are rooms over 7m2 and without water, i.e, you can exclude the kitchen and bathrooms but include lounges dining rooms, libraries, assuming they are bigger than 7m2.  The smallest tank you would have is 3000litres which is suitable for up to 5 main rooms. From the size of the tank the size of the filter system is calculated.

Septic Tank
This is the part that almost everybody has heard of, though its use is not always understood. This is where the “sludge” will remain, it is broken down by bacteria that are present, though it will never magically disappear, hence the enforced emptying of your tank every 4 years.

Epandage
In French it means spreading, although it is rather more refined than that.  This is the preferred method of disposing of the liquid exiting the septic tank: subject to the land being suitable.
There is no smell associated with it and it consists of a buried collection of pipes with holes sitting on some gravel, covered with a weed barrier and topsoil.
This Method is the cheapest but occupies the most land.  Which must be left as a lawn. Vehicles must not be parked on it. The epandage must be at least 5metres from the house and preferably any other buildings.

Sand Filter
There are 2 versions, a drained sand filter & non drained. A non drained filter might be used if the land drains well but there isn't enough space for a traditional epandage.  For a small house a filter 5 metres by 4metres could be allowed (some spanc will some spanc won't.... national standards eh!) although 5 by 5 is always preferred.
Most of the time a sand filter is used because the soil does not drain. The sand replaces the soil as a means to purify the liquid from the fosse septic. Because this sand is in the ground in a liner, the liquid is collected at the bottom and is, by preference, led to the closest ditch.  In the absence of a ditch there are other methods that are approved.  A drained sand filter should also be 5metres from the house although despite national standards some technicians will allow it closer to the house as a drained sand filter is a sealed system and will not soak the foundations with water.

Zeolite filter system
This is an approved compact filter system that only needs 15m2 for its installation.  If you lack the land for a system then this could be for you. Its drawbacks are it is a drained system which means that it needs a ditch to direct the exiting water into, although there are other ways that the water could be disposed of; This system, with larger component parts is now approved for properties  up to 20 rooms. This is the only compact system currently approved for properties this big.

Infiltration Well
Many people are told that all you need to do to “loose” the water that comes out of a Micro station is to dig an infiltration well. Well, this can be done, after a ground survey has proved that the ground below is suitable to absorb that amount of water per day and departmental permission has been obtained.  In reality it’s not very likely to be approved.

Carbon filters. humm....
Don't use them! they restrict the air flow through the system. If you have a concrete fosse the build up of gas will erode the concrete, if you lift the lid and you see white powder on the underside of the lid there is a problem with the ventilation. Make sure the the ventilation from the septic tank goes above the roof and that there is no bend sharper than 45 degrees. If the ventilation pipe is smaller than 100mm then that is part of your problem, already there is not  enough air flow nor does it  conform to current standards.

New systems approved for installation

Micro stations
They are often marketed as the panacea for all installations.
These are units that leave the liquid exiting from the unit as clean as is required by the French standards without having to use an epandage or sand filter to “improve” the liquid.  These units need to be maintained & emptied more often though some manufacturers claim three years between emptying. As SPANC visits are every 4 years emptying every two years would make sense. Micro stations need electricity to function as well., either to blow air into the sludge or turn a motor hidden within the tank.
The are several manufacturers of micro stations that have now been through the required test to get on the approved list,  If the unit is not on this list SPANC will not  approve its installation. Micro stations d'epuration can not be used for residence secondaire (holiday homes) as they all require electricity.  Because of the depth of these units their installation can be problematic here in the Creuse department. as rock is often present near the surface. If you have the space a classic system will always be cheaper.

Septodiffuseur.
This is another type of compact filter system that can be used in reduced spaces. It is an accordion type of non fibre material that is set on sand. This is approved for holiday homes as it doesn't use electricity but some of the S.P.A.N.C. technicians that I have spoken to claim that the filters clog up within a few years.  It has been suggested that the premature blockage has been due to inability to read the fitting instructions

Coconut Filters
These are compact systems that don’t use electricity with recycled coconut husks as the filtration medium. This system needs an annual contract where the husks are rotated (turned) and the exting liquid is analysed. From this test one can tell when the coconut husks will need replacing. The manufacturers suggest 10 years & this has a current cost of around 1200 euros, These systems can be used for holiday homes

Biorock  Systems
Again a compact system  that uses a filter derived from rock. Suitable for holiday homes up to 5 principle rooms. Information on the system seems rather sparse but one would expect it to have similar servicing criteria to a coconut filter system.

Pumps
As we all know water only flows down hill. This of course creates a problem if your septic tank is higher than your proposed WC. There are solutions, any of which are unfortunately expensive.  There are pumps that can lift “sludge” into a septic tank and others that will only lift liquid used after the septic tank, before the filtration system. If you have to have a pump in the system always try and put it after the fosse septique for when they need maintenance it is less messy!


DIY Soil Test
Dig a 30cm x 30cm hole x 50cm deep where you envisage putting the epandage. Fill it to the top with water leave until it has all infiltrated, then pour in 10 litres of  water and time how long it takes to be absorbed by the soil.  Ideally less than an hour but up to two hours and it could be possible to install an epandage system.  If the first test indicates an epandage is possible then do a couple more test holes by the same procedure to verify the porosity.  The seasons of course influence the grounds porosity.