I learned from the Australian breastfeeding association is that it is actually not necessary to sterilise your pump after every use. The reason is that breast milk is a living substance with live antibodies, enzymes etc. Hence, it will not grow bacteria unlike formula milk. Also, since the pump does not come into contact with baby's saliva, there is no risk of contamination, so no need to sterilise, unlike the teats/feeding bottles.
What you can do is to place the pump in the fridge at work (assuming there is a fridge) after the first pumping session, till your next pumping session. There is no need to wash it. Just pack it in a clean plastic bag and put place it in the fridge immediately after use. I have been doing this for the past 7 months while expressing at work and my baby has not fallen ill at all.
Really saves a lot of time and no need to feel guilty about not sterilising!
Somehow, I find that lactation consultants as well as the hospital staff in Singapore tend to over emphasise on sterilisation. A lot of them don't actually disseminate the correct information to new mothers and this creates a wide spread misconception that everything needs to be sterilised. Actually, only parts that come into contact with baby's saliva need to be sterilised.