A porous composite block is provided that contains at least two groups of binding agent particles. The first binding agent particles have an average particle size between about 5 .mu.m and 200 .mu.m. The second binding agent particles have an average particle size no larger than half the average particle size of the first binding agent particles and also between about 10 nm and 30 .mu.m. The porous composite block can also contain active grains and active granules such that the granules are smaller than the grains. The grains and the granules can be capable of treating water. A method of forming a porous composite block and a water treatment system are also provided.