After reading your question and the replies I somehow think that you are all missing the aim of a holding tank. The LAW requires you not to discharge grey water close to land and if you do so in the Med some countries will fine you very heavily if you even put bubbles in the water - do you want to swim in dirty washing up water? To stop this happening you need as large a holding tank as possible. In practice most of us livaboards discharge our waste over the side unless in a marina, harbour or swimming area. In this case we hold on to it until we leave (or discharge it illegally at night so that all traces have gone by morning but I will deny this fact in court!!). I have 25 lt. plastic tanks (vented to let the air out) for gally and shower which is enough for a day and a small, SWITCHED bilge pump discharges them . A diverter valve can be fitted to discharge directly over the side when away from land. As for cleaning, bilge cleaner once a year seems to work well.

Hope this brings and makes some sense.

David J