Theory :
The technique is to use a parent container that is shorter than the child element with scrollbar. This image shows what I mean :
Practice :
In your case, I suggest using absolute positionning and negative bottom value on #project-content so it overflows it's parent container (#projects) at the bottom.
The point is now what negative value? It should be the same value as the with of a scroll but scrollbars are never the same width according to browsers. So I suggest giving a bigger value : -30pxto be sure it is hidden. You will just need to be carefull that you don't have content to close to the bottom that can be hidden on browesers with thin scrollbars.
This is the CSS you should add to your website :
#projects{
position: relative;
}
#project-content{
position: absolute;
top: 0;
left: 20%;
bottom: -30px;
/* remove:
height: 100%;
position: relative;
float: left;
padding-bottom: -15px
/*
}