septic tanks ask an expert

I have a house on an estate which uses sewage holding tanks. All domestic waste water is routed into this tank therefore it cannot act as a "septic tank" because it takes bathroom and kitchen waste water as well. There is an overflow pipe which leads to a sewage treatment plant. There is no outfall or saok away. The only other method to empty the tank is pumping. However, between pumping intervals and despite having checked vents, lids and seals etc, there is a noticeable odor. There is no point in trying to treat the contents of the tank with any "biological" products as it contains chemicals like detergents which will negate such products. I would like to know what commonly available chemicals I could use to stop all activity and hopefully odours, stone dead in between pump outs. For example granular swimming pool chlorine, hydrochloric acid, hydrogen peroxide etc. Also what ratio should this be in? The tank holds 5'000 litres when full. It is plastic as are all connecting pipes. The tank is GRP and the pipes PVC.