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I'm not an expert on this but it seems that the 90 mm pipe is too big for the spring and far too big for the population to be served and would be a waste of money, unless the plan is to use water for irrigation. If the scheme goes ahead with 90 mm pipe it's always possible to put a gate valve on the discharge end to regulate the flow (or use a float valve in the tank) and have the pipe under pressure. If this is done, or smaller diameter pipe is used, then break-pressure tank(s) may be required as suggested, but that's not technically very complex although there would be some maintenance to plan. If open flow conditions are chosen it could be possible to deal with potential problems by placing some gate valves along the line to regulate flow and reduce likelihood of hammer and maybe some automatic purges to deal with airlocks at high points.

Another good reference would be the gravity flow chapter in the ACF manual 'Water, sanitation and hygiene for populations at risk', which is available on disk.

Regards,

John Adams