Headworks in a wastewater treatment plant (WWTP) is the initial set of processes that raw sewage undergoes upon arrival. This preliminary treatment stage is the facility’s first line of defense against large, problematic materials. The primary purpose of the headworks is to prepare the incoming water for all subsequent treatment phases. By removing the largest and most abrasive components, the headworks ensures the entire treatment system can function efficiently and without mechanical failure.
The Critical Role of Preliminary Treatment
Preliminary treatment is necessary because the raw influent contains items that would quickly damage or clog the sophisticated machinery used later in the process. Preliminary treatment focuses on removing non-biodegradable and abrasive materials, which are the inorganic components of the wastewater. This initial physical separation reduces the load on later stages, preventing the accumulation of heavy material in primary settling tanks. The headworks safeguards the entire plant’s economic and operational viability.
Screening for Large Debris
The first physical barrier the wastewater encounters is screening, which serves to capture large, visible debris. These materials, known as screenings, include items like rags, plastics, wood, and other non-biodegradable trash that enter the sewer system. Screening is categorized into coarse and fine stages, targeting different sizes of solids.
Coarse screens, often called bar racks, use widely spaced parallel bars (typically with openings greater than 6 millimeters) to remove the largest objects. Fine screens have much smaller openings (generally between 1.5 and 6 millimeters) and capture smaller debris like hair, coffee grounds, and disposable wipes. This finer screening is becoming more common due to the increasing volume of non-flushable products in the wastewater stream.
Modern facilities use automated mechanical rakes or brushes to continuously clean the screen surface and prevent clogging. Once removed, the screenings are transported to a handling system where they are often washed to separate out trapped organic matter, which is returned to the wastewater flow. Finally, the inorganic screenings are compacted to reduce volume and moisture content before being disposed of in a landfill.
Grit Removal Systems
Following the removal of large debris, the next step is to target smaller, denser particles collectively known as grit. Grit is composed of heavy, inorganic, abrasive materials such as sand, gravel, eggshells, and metal fragments. These materials are denser than organic solids, making them a threat to downstream equipment.
Grit must be removed because its abrasive nature causes significant wear on pumps and pipes, leading to high maintenance costs. If left unchecked, grit also settles in the bottom of settling tanks, reducing the effective volume and requiring costly periodic dredging. The removal process relies on manipulating the wastewater’s velocity to allow the heavy, inorganic grit to settle out while keeping the lighter, organic solids suspended.
Aerated grit chambers are a common system where air is injected into the water, creating a rolling, spiral flow pattern. This controlled turbulence washes the organic material off the grit particles and keeps the lighter organics in suspension, allowing the dense, clean grit to settle to the bottom. Vortex grit removal systems use a centrifugal force, where the water enters a cylindrical tank tangentially, forcing the heavier grit to the outer walls and down into a collection hopper. Both methods separate the dense grit from the water.
Flow Measurement and Control
The final component of the headworks involves monitoring and managing the volume of water entering the treatment facility. Accurate flow measurement is necessary for operational control, regulatory compliance, and proper chemical dosing in later stages, providing operators with the real-time data needed to adjust processes.
Commonly used devices for measuring flow in the headworks include Parshall flumes and magnetic flow meters. A Parshall flume is a fixed, open-channel structure that restricts the flow, allowing the water level to be measured and converted into a flow rate. Magnetic flow meters use an electromagnetic field to measure the velocity of the conductive wastewater, providing a highly accurate measurement without obstructing the flow path.
The headworks also incorporates mechanisms to manage hydraulic variations, particularly during heavy rainfall events that cause significant spikes in flow. Equalization basins may be integrated into the headworks area to temporarily store excess flow, allowing it to be treated at a steady rate. If the flow exceeds the plant’s maximum capacity, bypass mechanisms are used to divert the excess water (usually after screening) to prevent flooding of the plant’s interior processes.