Something to consider in your risk analysis. Under the Clean Water Act, fecal coliform is a pollutant, and intentionally dumping it on the ground is a "release". Now, I don't know if dumping it on the ground is illegal in all circumstances, or only in certain circumstances. That determination may be fact specific. The state governments also have a hand in permitting septic tanks (i.e., fees).
The problem is, if your dumping is illegal and you get caught and the government requires remediation, the cost of a full blown aeration septic system might very will look cheep in hindsight.
Also if the owner wants to sell the property in the future and an environmental inspection is required, the inspectors will have a heyday with you and the owner if they detect illegal dumping.