3.8 Lagoons, Stabilization Ponds & Natural Treatment Systems

Lagoons, Stabilization Ponds & Natural Treatment Systems

Table 1 of 9VAC25-790-290 classifies natural treatment methods separately from biological mechanical and advanced waste treatment, and it places natural systems above 1.00 MGD at Class III and natural systems from 0.001 through 1.00 MGD at Class IV. A large share of Virginia's smaller works are ponds, and the ABC Wastewater outline lists "aerated lagoons or constructed wetlands" and "processes utilizing land treatment" among its biological process families.

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1. Pond Types

The facultative pond's layered structure

1. Aerobic surface layer. Algae photosynthesize during daylight, producing oxygen; wind mixes oxygen into the top foot or two. Aerobic bacteria consume soluble BOD.
2. Facultative middle zone. Oxygen present intermittently; facultative organisms switch between aerobic respiration and fermentation.
3. Anaerobic benthic layer. Settled solids undergo anaerobic digestion, releasing methane, carbon dioxide, ammonia, and hydrogen sulfide, plus soluble organics that recycle upward.

The relationship is symbiotic: algae supply oxygen to the bacteria, and bacteria supply carbon dioxide and nutrients to the algae.

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2. Loading Rates

Organic loading on ponds is expressed per unit area, not per unit volume, because oxygen supply is a surface phenomenon.

Organic loading (lb BOD/day/acre) = Flow (MGD) x BOD (mg/L) x 8.34 / Surface area (acres)

Typical ranges:

Hydraulic loading is expressed as detention time:

Detention time (days) = Pond volume (gal or acre-ft) / Flow (per day)

Worked example

A facultative pond is 4.5 acres at an average depth of 5 ft, receiving 0.22 MGD at 190 mg/L BOD.

1. BOD load = 0.22 x 190 x 8.34 = 349 lb/day
2. Areal loading = 349 / 4.5 = 77.5 lb BOD/day/acre - substantially overloaded for a temperate facultative pond, which predicts odor, loss of the aerobic surface layer, and possible pond turnover.
3. Volume = 4.5 acres x 5 ft x 43,560 sq ft/acre x 7.48 gal/cu ft = 7,330,000 gal
4. Detention = 7,330,000 / 220,000 = 33 days

The detention time is adequate but the areal loading is not - a good illustration of why both must be checked.

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3. The Algae Problem

Ponds routinely achieve excellent soluble BOD removal and then violate their TSS permit limit. The reason is structural: the algae that oxygenate the pond are themselves suspended solids. Effluent TSS from an unpolished facultative pond commonly runs 40 to 100 mg/L, of which most is algal cells, and those cells do not settle in a conventional clarifier because they are buoyant and motile.

Standard responses:

• Effluent filtration - sand, cloth media, or rock filters.
• Baffling and a quiescent final cell to encourage settling.
• Dissolved air flotation - effective because algae float readily.
• Chemical addition (alum or ferric) to coagulate algae ahead of a filter.
• Intermittent sand filters or a constructed wetland as a polishing stage.
• Seasonal or controlled discharge - hold effluent during algal bloom periods and discharge during high stream flow. Virginia permits at some pond systems allow this.
• Depth control - withdrawing from below the algal layer rather than at the surface.

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4. Operating a Pond System

Diurnal chemistry

Photosynthesis consumes carbon dioxide, which drives pH up during the day. At night algae respire and consume oxygen. The result is a pronounced daily cycle:

Sample at a consistent time, and understand that a pH violation at 3 p.m. in July is an algae story, not a chemical feed story. High afternoon pH also drives ammonia toxicity, because the fraction of un-ionized ammonia rises sharply above pH 9.

Seasonal turnover

Like a water supply reservoir, a deep pond stratifies in summer and turns over in spring and fall. Turnover mixes the anaerobic benthic layer through the water column, producing an abrupt odor event, a black or gray appearance, a dissolved oxygen crash, and an effluent BOD and TSS spike. Operators anticipate it, and mitigate with supplemental aeration, sodium nitrate addition, or recirculation.

Routine operator tasks

• Level control across multiple cells - series operation for treatment, parallel for load sharing, and the ability to switch is the operator's main process lever.
• Dike maintenance - mow to control burrowing animals and to permit inspection; riprap the waterline; watch for seepage on the outside toe, which signals a developing breach.
• Weed and emergent vegetation control - cattails at the edge provide mosquito habitat and block wind mixing.
• Sludge depth survey - measure benthic accumulation every 3 to 5 years with a sludge judge or sonar. Ponds are typically desludged every 10 to 20 years; accumulated sludge reduces volume and detention time long before anyone notices.
• Short-circuiting control - dye testing reveals flow going straight from inlet to outlet. Baffle curtains and relocated inlets restore effective volume.
• Odor management - the primary complaint driver. Sodium nitrate provides an alternate electron acceptor and suppresses sulfide; surface aerators break stratification.

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5. Aerated Lagoons

An aerated lagoon abandons photosynthesis as the oxygen source and supplies air mechanically, which decouples treatment capacity from surface area.

• Partial-mix (facultative aerated) lagoons supply enough energy for oxygen transfer but not enough to keep solids suspended, so solids settle and digest on the bottom. Typical power is 10 to 20 hp per million gallons.
• Complete-mix aerated lagoons keep all solids in suspension, typically at 50 to 60 hp per million gallons, and require a downstream settling cell.
• Aeration equipment is either floating surface aerators (simple, prone to icing and to spray drift) or diffused air on weighted laterals (better transfer efficiency, harder to service).
• Cold weather is the design constraint in Virginia's western counties. Biological rates roughly halve for each 10-degree-Celsius drop, and surface aerators ice up. Winter detention time must be long enough to compensate.

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6. Constructed Wetlands

Table 1 lists "aerated lagoons or constructed wetlands" together as a biological method.

Operating notes:

• Wetlands are polishing processes. Loading a wetland with raw or primary effluent clogs it.
• Nitrogen removal occurs through nitrification in aerobic root zones and denitrification in anoxic bulk media, and is seasonal.
• Phosphorus removal is limited and declines over time as media adsorption sites saturate - a wetland is not a long-term phosphorus sink without media replacement.
• Vegetation management, water level control, and inlet distribution uniformity are the recurring operator tasks.

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7. Land Treatment

Table 1 lists "processes utilizing land treatment" with the attendance note "during land application." Three modes:

1. Slow rate (irrigation) - effluent applied to vegetated land at agronomic rates; treatment by soil filtration, adsorption, and plant uptake. Loading of a few inches per week.
2. Rapid infiltration - high loading onto permeable soils in basins, with treatment by percolation; groundwater recharge is the disposal route.
3. Overland flow - applied at the top of a graded, vegetated slope over relatively impermeable soil, collected in a toe ditch.

All three require storage for periods when application is impossible - frozen ground, saturated soil, or crop cycle - and Virginia's wet winters make storage volume the controlling design parameter. Operators track application rate, soil moisture, groundwater monitoring wells, buffer distances, and nitrogen loading against the agronomic rate.