This is one of the best comprehensive wastewater treatment books for anyone designing wastwater treatment facilities on a smaller scale, as well as planning on a larger scale.
The biggest downside to the 4th volume of the book is that while it says it supports a dual unit system, much of the book is exclusively in SI, which has not yet taken quite enough hold in the US to make it efficient. The biggest plus of the 4th volume is that some newer treatment technologies are discussed in more detail than in the 3rd edition.
This is a must have for a Sanitary Engineer! Anyone interested in wastewater treatment would also benefit from this text. The book provides a deep understanding of the biology of wastewater treatment as well as the varying treatment processes.