SpecialFocus

By Mike Oliveto, Mitsubishi Chemical Advanced Materials

Given that more than 70 percent of the earth is covered by water, we all have a role to play in keeping our water clean and healthy enough to sustain future generations. We have all seen photos of lifeless rivers and lakes, consumer plastics washing up on beaches and closed beaches resulting from both clearly identifiable sources such as pipes or culverts carrying water from homes or industrial sites and not so easily identifiable sources such as diffuse flows running over or through land. An example of the latter is fertilizers, fecal matter from farm animals and even salt used to treat roads washing into waterways during heavy rains and snow melts. The source of these pollutants is often difficult to detect and control because they can be spread out over wide areas and may remain invisible for years until concentrations reach critical levels.

Governments, cities, municipalities, major manufacturing businesses that use large amounts of water are required to clean water before returning it. We call the cleaning process wastewater treatment. In simplest terms a highly engineered system brings wastewater in, cleans it using a process sequence that includes the settling of solids (sedimentation) and biological and chemical treatment prior to the water being returned either to the process consuming water or as an outflow completing the water cycle.

Schematic courtesy of Saltworks Technologies.

Engineering plastics are important in such systems replacing traditional metals such cast iron and bronze improving the life of these parts, reducing the energy consumed by these systems while eliminating the risk of further contamination from metal corrosion over time. Although the environment in these systems varies based on the water entering the system, nylon and ultra-high molecular weight polyethylene (UHMW-PE) are the most commonly used wear materials while thermoset pultrusions are commonly used for high stiffness structural parts.

The most common fit for these engineering plastics is in the clarifiers where particulates are removed from water via sedimentation. We have all seen these systems, which can be round or rectangular.

Despite what seems at first glance to be static systems, these clarifiers have many moving parts including: scrapers or flights that drag along the tank bottom; sprockets and chain that move the flights, chain tensioners, shafts and wear shoes that provide wear and abrasion resistance and a collector that skims floatable solids from the surface of the water in the tank.

Below is a typical set up for a rectangular clarifier. All components within the tank are today plastic, eliminating corrosion while reducing the weight and resultant stress on all parts of the system. These parts spend most of the time below the surface of the water and are not visible when the systems are functioning.

A rectangular clarifier made by Brentwood Industries (Polychem).

Why nylon and why UHMW-PE for such wear components? UHMW-PE and nylon are ranked first and second respectively in terms of the abrasion resistance via sand slurry testing. Nylon’s greater strength and stiffness (3-4 times that of UHMW-PE) and ability to be cast into large parts make it ideal for large sprockets and stub shafts that sprockets are mounted on. Chain segments are produced in such large numbers they are mostly injection molded from a reinforced nylon while wear strips mounted along the tank bottom are almost always virgin UHMW-PE. The flights that span the tank width are mostly fiberglass reinforced plastic (FRP) with the blue wear shoes mounted to them prevent premature wear of the FRP.

Opportunities for serving this industry exist at OEMs and individual wastewater treatment facilities. Do not be tempted to suggest other materials for such wear parts. Although acetal (POM) or polyester PET may do better than nylon in a wet food industry application, they will not survive the abrasive conditions in these systems. High-density polyethylene (HDPE) or polypropylene (PP) may offer improved cost, inertness and weldability over UHMW-PE but will again not provide the abrasion resistance required by parts operating within such systems.