Texas Waste Water Treatment Ch#07 Suspended Growth Systems (ponds)

The Texas design criteria (30 TAC §217.201) provides standards for ponds of various depths and methods of aeration. Ponds are classified as the following: Wastewater stabilization ponds (secondary treatment ponds), aerated lagoons, facultative lagoons (raw wastewater stabilization ponds), anaerobic lagoons, and hyacinth ponds.l

All pond systems are capable of reducing biochemical oxygen demand (BOD), bacteria, and other biodegradable materials.

Ponds may also be classified according to dissolved oxygen content. An anaerobic pond lacks oxygen most of the time because the oxygen demand is much higher than the oxygen supply. In a facultative pond, the upper portion has oxygen, while the bottom layer is anaerobic. Oxygen in an oxidation or aerobic pond (also known as a wastewater stabilization pond) is distributed throughout the entire depth most of the time. Because sunlight must penetrate the entire depth of an aerobic pond, it is shallower than an anaerobic or a facultative pond.

A typical waste water stabilization pond is designed to be aerobic throughout its depth. As solids accumulate in the pond only the sludge layer will become anaerobic. Aerobic ponds normally have a water depth of 3 to 5 feet.

Wastewater stabilization ponds follow primary treatment, serving as secondary treatment. State regulations require multiple ponds, which provide the flexibility to allow the operator to take one pond out of service without affecting the remaining ponds. Routinely, multiple ponds are operated in series.

The answer key is wrong on this one but I'm told it'll be the answer on the licensing exam so I'm not changing it however the text book has this to say on the matter: "Multiple outlets and inlets prevent short-circuiting, assuring even flow and adequate detention time through the entire length of the pond." ". Inlets are submerged and evenly spaced. Outlets are constructed with an adjustable baffle to keep floating material out of the effluent. Submerged outlets should be used to minimize the discharge of floating algae."

Organic loading on wastewater stabilization ponds (when used for secondary treatment) is based on the total surface area of all ponds and should not exceed 35 lb. BOD5/acre/day. Detention times are above 30 days. The loading on the initial pond should not exceed 75 lb. BOD5/acre/day.

Facultative organisms will adjust their metabolism to the oxygen available at the wastewater plant. They prefer to use free or dissolved oxygen when it is available, but will shift to an anaerobic way of living when free oxygen is not present. Facultative lagoons have two zones of treatment, an aerobic surface layer and an anaerobic bottom layer. Facultative lagoons operate with 5 to 8 feet of water depth and are usually loaded organically up to 150 lb. BOD5/acre/day. They are designed to provide natural aeration to maintain aerobic conditions in the top layer.

Anaerobic lagoons receive raw, untreated wastes. There are usually two or more individual ponds; the first one is the primary pond and the other(s) are secondary ponds. These ponds may be operated in series or in parallel. Texas municipalities do not usually use anaerobic ponds, but they can be found in industry, livestock feedlots, and meat-packing operations.

Wastewater stabilization ponds follow primary treatment, serving as secondary treatment.They stabilize organic wastes through a complex, natural process involving sunlight, atmospheric oxygen, water, nutrients, algae, and bacterial action.

Wind speed and direction are important to pond operation because the interaction of the water surface and the atmosphere dissolves oxygen in the water. The ratio of surface area to volume and the turbulence provided by wind action are also important in atmospheric aeration. Prevailing winds in Texas are from the south-southeast. Long, narrow ponds are preferable and should be oriented in the direction of the prevailing winds so that debris is blown toward the inlet.

Ponds depend on adequate detention time. Short-circuiting occurs when the flow is not evenly distributed or when the flow moves in a narrow zone or channel. Short-circuits reduce the detention time and treatment efficiency.

A wastewater pond influent contains unstable organic matter and nutrients. Aerobic bacteria in the pond use oxygen to break down organic matter, resulting in the release of carbon dioxide. The carbon dioxide becomes a nutrient source for algae and is converted to simple sugars through a process called photosynthesis. In photosynthesis, sunlight provides energy needed by algae to split water into hydrogen and oxygen. The hydrogen combines with the carbon dioxide to create the glucose molecule (sugar), while the oxygen is released as a by-product. Light-absorbing pigments in the chloroplasts of algae cells use sunlight to convert water and carbon dioxide into chemical energy through the chemical reaction of photosynthesis. 6H2O + 6CO2 -> C6H12O6 + 6O2 Aerobic bacteria use free oxygen produced by photosynthesis to stabilize more organic matter in a continuous cycle.

Good housekeeping is a must. Dikes and the surrounding area must be kept free from weeds, brush, and other obstructing vegetation. Cattails and other shallow-water plants must be eliminated. Shallow and shaded areas are breeding places for mosquitoes and must be cleared.

An adequate testing program for process control is necessary. The quantity and quality of both influent and effluent must be known. Measurements are taken at the influent point, in the pond itself, and at the point of effluent. Several tests may be run on samples from each of these three major points, including the following: Flow, temperature, pH, dissolved oxygen (DO), biochemical oxygen demand (BOD or BOD5 when referring to this test), total suspended solids (TSS), and chlorine residual.