Septic systems are now designed specifically for the site based upon daily design flow, soil borings, and an on-site study of the property. The reason your replacement system must now have a bigger absorption field, a system with a dose tank and pump, a larger septic tank, or a mound system is because that is what the soil and conditions on your property qualify for. Most likely, if the existing system has been in use for many years, it was not designed; it was just the standard version of septic systems at the time of installation. Furthermore, due to lack of space, water wells, the addition of fill, compaction, and landscaping, soil and site conditions are different than when the property was developed. If you have questions about system sizing or type, contact an environmental health specialist at 235-9721 for further explanation.