Appropriate devices for obtaining either interstitial water or grain particle samples or for measuring dissolved oxygen (DO) at depth within the interior of large-scale, industrial granular bed filters are currently lacking. We developed two innovative samplers for studying a Rapid Sand Filter (RSF) at an operational SWRO plant: the first for water and granular material, the second for DO measurements. 1. The particle and interstitial water sampler is based on a corer that retains grain particles inside a chamber using water suction created by an external peristaltic pump. 2. The DO sampler incorporates a temperature compensated oxygen measuring optode over which laminar water flow is maintained by an external peristaltic pump. To test these samplers, an operational, full-scale RSF at a SWRO plant was monitored at various depths within the filter bed over several months. Additionally, DO profiles within the filter bed were followed during and after a backwash cycle. These samplers enable efficient and precise collection of DO data, interstitial water, and granular material from previously inaccessible depth layers within the filter bed without interruption of the filter operation, providing information on microbiological processes such as biofilm development, microbial community structure and organic matter cycling.