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Introduction

Lagoons are one of the oldest wastewater treatment systems. They consist of inground, earthen basins where wastewater is received, held for a period of time, treated, and then discharged (Fig. 1.1). Depending on the composition, strength, and volume of wastewater to be treated and discharge requirements, a predetermined "hold" or retention time for the wastewater is used. Although lagoons are simple in design, there are many complex biological, chemical, and physical processes that occur in lagoons. There are several different terms for lagoons. Many of these terms are used interchangeably. These terms include pond, oxidation pond, polishing pond, sewage pond, stabilization pond, maturation pond, and cell. Some terms refer to the role of the lagoon in a wastewater treatment process (Table 1.1).

Figure 1.1 Facultative lagoon. A facultative lagoon has three active microbial zones where wastewater is treated. The zones consist of aerobic, facultative, and anaerobic habitats.

Table 1.1 Terminology for lagoons or ponds

Term Description Maturation Improves effluent from activated sludge process or trickling filter to tertiary quality, principally to reduce the number of pathogens and nutrients Oxidation Wastewater is purified by sedimentation and aerobic and anaerobic treatment Polishing Increases the environmental quality of effluent from previous treatment Primary Receives raw (unsettled) wastewater and has a sludge layer that is responsible for methane production Secondary Receives settled wastewater or effluent from an anaerobic lagoon Stabilization An anaerobic, facultative, or maturation lagoon

Lagoons or natural impoundments were used in the United States in the 1920s to capture liquid wastes. With increasing knowledge of the wastewater treatment ability of lagoons, they were and are used to treat agricultural, domestic, industrial, and municipal wastes and wastewaters.

Lagoons became popular in the 1950s. Today, numerous lagoons and lagoon systems are used, especially in rural areas for the treatment of domestic and municipal wastewaters. Although performance varies from good to bad, lagoons that are properly designed, constructed, and operated can produce effluent that meets secondary treatment standards.

Lagoons are one of the most popular, simplest, and least expensive technologies for treating wastewater. Lagoons do require relatively large amounts of land. For each million gallons per day (MGD) of wastewater, approximately 30 acres of lagoon are required for 50 pounds biochemical oxygen demand (BOD) per acre per day. Some "once-through" lagoons are as large as 40 MGD.

Lagoons use natural and energy-efficient processes to provide low-cost wastewater treatment, and they offer an alternative to more advanced wastewater treatment processes. Although lagoons offer many advantages such as reduced maintenance cost when compared with other wastewater treatment systems (Table 1.2), they also have disadvantages such as the requirement for large amounts of land (Table 1.3).

Table 1.2 Advantages of lagoons as compared with more advanced wastewater treatment processes

Can handle intermittent use and shock loads better than other processes Inexpensive to construct Relatively small quantity of equipment is needed Reduced maintenance costs Relatively simple to operate Effective in removing pathogens Effluent often suitable for irrigation due to high nutrient content Fewer solid-handling problems

Table 1.3 Disadvantages of lagoons as compared with more advanced wastewater treatment processes

Less efficient in cold climates than other processes Require large amounts of land Effluent quality varies with seasonal changes in wastewater temperature Effluent from facultative lagoons contains algae and may require additional treatment to meet discharge requirements Seasonal turnover and release of benthic organisms System upsets usually result in odor production If not properly maintained, lagoons can provide a breeding area for midges, mosquitoes, and other insects

Lagoons are designed to satisfy a specific site and need. The design is based on several factors including type of soil, amount of land area required, climate, quantity and composition of the wastewater to be treated, and discharge requirements. However, lagoon effluent may require additional treatment or polishing to remove pathogens or nutrients.

Most lagoons are found in small rural communities. Here, lagoons often cost less to construct, operate, and maintain than other wastewater treatment systems. Although lagoons require more land than other wastewater treatment systems, land is usually more available and affordable in rural areas. Because lagoons treat a large variety of wastes, they must be properly constructed to prevent soil and groundwater contamination.

Wastewater lagoons must be sealed or lined to prevent seepage at the bottom and sidewall of the lagoon to prevent subsurface and groundwater contamination. There are several types of liners that are used: (i) clay, cement, and asphalt; (ii) synthetic and rubber; and (iii) natural. Most liners typically perform well for 15 years. However, premature failure can occur and is usually due to (i) cleaning or dredging operations, (ii) membrane puncture, (iii) scour of cover material, (iv) substandard liner material, and (v) weed growth. The most commonly used liners for industrial wastewater lagoons include (i) chlorosulfonated polyethylene or Hypalon®, (ii) polypropylene (rPP), (iii) polyvinyl chloride, (iv) reinforced liner-low density, and (v) XR-5®/XR3®. These liners are tolerant of ice buildup and exposure to harsh and prolonged sunlight.

Clay liners shrink and swell according to wastewater temperature and wet-and-dry conditions. However, compacted clay liners are susceptible to erosion and vegetative growth in the dike. These conditions damage the integrity and strength of the liner and dike.

Installation of clay requires proper moisture content and compaction. Cement and asphalt liners can crack under temperature change and wet-and-dry conditions. Synthetic liners are commonly used and usually consist of some type of plastic. They require careful installation by an experienced contractor. If the liner is properly installed and is not punctured, seepage does not occur. Synthetic liners are inert and therefore, they are often used in lagoons that contain toxic wastes.

The clogging of soil pores forms natural liners. This occurs due to (i) settled solids, (ii) microbial growth, and (iii) chemical clogging of the soil due to ionic charges. However, natural liners tend to be unreliable because these natural modes for forming a natural liner or sealant are dependent on changing characteristics of the wastewater.

Large and diverse populations of archaea, algae (Fig. 1.2), bacteria, and protozoa (Fig. 1.3) are found in lagoons. Changes in numbers and dominant groups or species of organisms depend on biotic (biological) and abiotic (chemical and physical) factors (Tables 1.4 and 1.5). The most important abiotic factors are (i) composition and strength of the influent, (ii) dissolved oxygen, (iii) pH, (iv) temperature, and (v) sunlight.

Figure 1.2 Commonly occurring algae in facultative lagoons. Commonly occurring blue-green algae or cyanobacteria in facultative lagoons include (a) Phormidium, (b) Aphanotheca, (c) Planktothrix, and (d) Aphanizomenon.

Figure 1.3 Commonly occurring protozoa in facultative lagoons. Commonly occurring protozoa in facultative lagoons include the amoebae Arcella, Amoeba, and Difflugia; the flagellates Bodo and Pleuromonas; the free-swimming ciliates Paramecium and Strongylidium; the crawling ciliates Euplotes, the stalk ciliates Epistylis and Opercularia; and the suctorian Acineta.

Table 1.4 Biotic factors affecting changes in numbers and dominant groups of organisms in facultative lagoons

Algal growth Available niches Burrowing animals Carbon dioxide production Cell lysis Denitrification Dominant microbes Food webs Methane production Mixed acid...