Ultrathin and ultrasmooth ZnO films, grown by atomic layer deposition (ALD), are used as seed layers for chemical bath deposition (CBD), to synthesize dense and long ZnO nanorod arrays. The enhanced photocatalytic capability of the sintesized materials reveals their potentiality for wastewater treatment. The thickness of the ALD seed layers triggers the final ZnO nanorods height and density and, accordingly, the overall photocatalyst surface area. In particular, the thinnest ALD ZnO layer promotes the grow of highly oriented and dense ZnO nanorod arrays, which are able to efficiently degradate methylene blue dye and phenols under UV light irradiation. These heterogeneous photocatalysts are stable to aging effects, can be easily removed from water without any filtration process and can be efficiently reused for multiple degradation cycles.