If its a closed and non ...

If its a closed and non ferrous trench then ozone works well by disintegrating most of the solid contaminants. Ozone has been tried to reduce the TSS load. There are no residues of ozone but oxygen hence no addition in COD/BOD. Moreover the slimy layers created by microbes is a major issue in gravel trenches, such layers are killed by ozone. If it is an open trench, you can arrange simple Ozone bubbling at various points through a small pipe burried under gravel with multiple holes at small distances. This will solve your problems completely for long term without chlorine and chemicals with added advantage of reduced or disintegrated solids. No foul smell is a bonus!