Hi Carol,
Under no circumstances can the black tank valve be left open and it be used. Large amounts of liquid are required to move solid waste out the holding tank, through the trailer waste plumbing, and out the hose to the dump.
Since the trailer waste path is a vertical drop into a tank and then out the ribbed hose rather than a smooth, gradual path, the solid waste would be left behind if it did not have the large amount of liquid in the tank to carry it out. If the valve was left open, it would ultimately form a clog in the black tank that is a real pain to clean up.
Everyone here will have an opinion on what works best for odor control. We use Tank Techs RX and have excellent results. Go to their web site, read all the info, and decide for yourself if it is the product you want to use, it is unlike anything else on the market. When used regularly according to the directions, it will control odor and leave your tank cleaner than any other treatment available. Their web site is
http://www.tanktechsrx.com
Dave
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Dave, Barb, and Paisley the Wonder Wiemeraner
Was - 2015 3160RL Legacy Edition & 2014 F350 6.7l PSD SRW Platinum
Now - SOB