PP probably means polypropylene (one of about a billion types of plastic now in use). HDPE (high-density polyethelene) has a higher melting temperature than PP. I wouldn't think this would be an issue, but be careful if your placing heaters in this sump. The sump may need some additional bracing on the corners, front, and sides to prevent a lot of bowing from the water pressure. IMHO, it wouldn't be a problem to use this though, as long as the water doesn't get real hot (heater malfunction).
