Management of significant industrial users (SIUs) has been an important part of wastewater treatment plant operations since the 1980s, when new plants began experiencing process upsets due to industrial discharges. LRWWU has had an EPA-approved Industrial Pretreatment Program (IPP) since 1998. The purpose of the IPP is to prevent pass-through and interference caused by industrial discharges. 

LRWWU’s diligence with pretreatment activities can be credited for its success in avoiding process upsets and non-compliant discharges to the Merrimack River. Over the years, the number of SIUs has declined slightly, as domestic manufacturing has moved overseas.