Control of Substances Hazardous to Health Regulations.
Possible stagnant water is a bit extreme but can you dismiss it?
Softener and soap suds, again a bit extreme but can it be discounted?

Manual Handling Operations Regulations.
Water is a hazard as it has no designated centre of gravity…….it flows, moves and it’s heavy. Added to the spin counter-balance in a washing machine (often a block of concrete) = “One slipped disc to go please”.

Workplace (Health, safety and Welfare) Regulations. No matter that it is a domestic property, the workplace regs still apply to your people.
Slips, trips and falls on the water and trailing cables etc.

Management of Health and Safety at Work Regulations.
No risk assessment? I would say that it was a significant risk. And if you have got 5 or more employees then it must be in writing.

Health and Safety at Work Act 1974.
Section 2,2 safe place of work etc etc.
Section 3 your employer must conduct their activities in such a way that they do not put others at risk. If a householder slipped on the water that your people left, even though it’s on their premises then you may be found wanting. Spill it outside on the path in winter and you could have black ice.
Also civil legislation comes into play here under vicarious liability.

Just remember that you have no option but to collect under the WEEE Regulations 2006. You need to find answers. To be honest though once water inlets and outlets are sealed there is little risk for your people - the recycling company have the real problems.

Bob