Septic tanks or package treatment plants, rely on natural processes and bacteria, to break down sewage, and purify the final effluent to make it suitable for discharge. You can help this process by being careful what you put into the tank. Bear in mind, anything manmade, is probably going to take a lot longer to decompose, if it'll decompose at all. Condoms, sanitary products, cotton buds, etc, will not break down and just serve to bung the thing up. Detergents can affect the bacteria levels, and thus the operation of the tank. Look after it, and it'll look after you, and give many years of trouble free operation.
If you're still unsure, (and I would want a second opinion before parting with £10K), have a look at Klargester, they may be able to advise of an approved contractor in your area that could have a look and offer a second opinion. https://www.kingspanenviro.com/kingspan-klargester-wastewater
If you're still unsure, (and I would want a second opinion before parting with £10K), have a look at Klargester, they may be able to advise of an approved contractor in your area that could have a look and offer a second opinion. https://www.kingspanenviro.com/kingspan-klargester-wastewater