• Engineering Waste Water Treatment Systems that Work

    11/25/2012 12:42:47 PM
  • The keys to designing reliable waste water treatment systems are:

    • Understanding the influent wastewater, and
    • Understanding the limitations of treatment components.

    This is especially true in small treatment systems where flows and loading can be highly variable. Although the wastewater industry considers a small system those with flow less than 100,000 gallons per day (gpd) or 380 cubic meters per day (m3/d), very small systems from about 35,000 gpd (130 m3/d) and below require increasing understanding of water quality and flow to be successful.

    Very small wastewater treatment systems usually have a much higher range of hydraulic and organic load variability than that taught in engineering courses or found in textbooks. There is little specific design guidance found in traditional sources for very small systems. Sources such as the National Small Flows Clearinghouse (NSFC) are helpful, but specific design guidance and experience is slim.

    Small waste water treatment systems cannot be designed as scaled-down versions of large waste water treatment plants. This, combined with experience of the regulatory process, helps to choose and design an effective and efficient wastewater treatment system.

    Design Standards

    In other words: What are the Goals?

    Generally, regulatory limits define the treatment goals. Wastewater treatment systems are designed to meet certain hydraulic, nutrient, and other limits based on regulatory standards.

    Wastewater systems are regulated by USEPA, State, or local agencies depending on the system’s jurisdiction location and the size of the wastewater discharge. Each of these agencies has specific limits as to the quantity and quality of water that may be discharged. Regulation of wastewater discharge criteria is based on a permit that may provide for daily, weekly and annual limits.

    Determine (or Negotiate) the Treatment Goal

    A difficult ruling from a State agency regarding wastewater treatment standards can make or break a facility. This is becoming a greater factor in commercial and industrial facility operations as the State becomes aware of new relationships between high strength wastewaters, the soil, and groundwater.

    For example, a beverage company located for many decades in Paw Paw, Michigan produces high BOD effluent due to sugars in the wastewater. In the last few years, it has learned that it will have to treat its wastewater prior to land application due to recent understanding that high BOD wastewater can leach iron, manganese and arsenic into the groundwater.

    Complex wastewaters and complex hydrogeology can turn the process of selecting the right treatment standard into a series of negotiations. State discharge standards are fairly clear on the surface, but some discharge locations have complex hydrogeology which makes “standard” discharge standards inappropriate. It’s important for facility owners to make sure they’re getting the treatment standard that’s appropriate for their wastewater and their site.

    Wastewater Quality and Wastewater Flow Rates

    In other words: Know Thy Wastewater

    This is probably the single most important aspect of designing a wastewater treatment system. If the wastewater influent is not known well, it can reduce even the best designed, sewage treatment system into an overpriced series of junk with nothing more than an enforcement letter from the State to show for it.

    You can have a book of data to show but if you don’t really understand how the effluent is produced and when the effluent is produced, then you’re in trouble. An engineer cannot take engineering “rules of thumb” that work for large systems and apply them directly to very small systems.

    Two factors of wastewater need to be understood: Hydraulic loading and organic loading (industrial wastewater, depending on the industry, may need to consider inorganic loading as well). Hydraulic loading is essentially the flow of water into a wastewater treatment system at any point in time. It has found that in small wastewater treatment systems, flow rates can vary from nearly zero to a maximum flow that exceeds 10 times the average flow. As a comparison, flow in large wastewater treatment plants varies from 2 to 5 times average and never encounters a flow of zero. 

    Producers of Highly Variable Flows

    Flow rates that vary a factor of 10 or more on a weekly or daily basis are not uncommon in very small waste water treatment systems. Waste water treatment systems that receive highly variable flows are generally those that are not part of a community or municipal system, receive water from a single business or use, and sites of single land use. Examples are:

    • Resort uses such as waterfront hotels, golf resorts, and alpine ski resorts,
    • Small residential neighborhoods and condominiums,
    • Fruit and vegetable processors, and
    • Certain types of industrial operations.

    The consequences of highly variable flow are significant to a waste water treatment system. Because the treatment system is largely a biological process, variations in flow disturb the bacteria population. A period of low or zero flow in a treatment system will soon starve good bacteria of their food supply. When a surge of high flow occur, such as when a resort maxes out occupancy when the weekend starts, or when a fruit processor starts up operation after the weekend ends, the bacteria cannot multiply fast enough to adequately consume the incoming organic load unless components designed for these variations are put in place.

    Choosing a Waste Water Treatment System

    The effect of highly variable hydraulic and organic loading on waste water treatment system effectiveness varies based on the type of system treating the waste water. Lagoon systems are very effective at handling highly variable waste water influents. However, lagoon systems use up more land area than other systems and may not be practical on smaller sites. On the other hand, advanced, onsite wastewater treatment systems may use less land, but usually use more energy to operate.

    There are several options available for the selection of the right components and systems for the design of an effective treatment system.