Elimination of eutrophication through resource recovery

James L Barnard Black & Veatch 8400 Ward Parkway, Kansas City MO 64114

Abstract The undiminished growth in the world population and the spread of industrialization to former agricultural societies have put a relentless pressure on resources in terms of not only the supplies of food and fuel but also of the rejected energy that can cause serious pollution of receiving waters. This paper will look at the causes of deterioration of the water environment and how this resource can be recovered while solving many of the pollution problems. Proteins can be recovered from wastewater to augment the food supplies, urine can be separated and used as fertilizer, phosphorus can be recovered and used in fertilizers or incorporated in compost, algae can be grown and harvested for converting to bio-diesel fuel, wastewater biosolids can be turned into organic fertilizers and water can be reclaimed for re-use which would prevent deterioration of receiving water quality.

INTRODUCTION

Over the past 35 years the population of the world has doubled, from roughly 3 billion to about 6 billion. Since land is at a premium in most high-growth countries, most of this growth was in the cities. It is expected that by 2050, the world population will stabilize at around 10 billion. The growth will not be spread uniformly around the globe but will be concentrated in countries that are already disadvantaged. The concentration of people leads to concentration of pollution and the need for treatment of the wastewater generated in the urban areas occupied by this population. As an example, the population of Mexico City now exceeds 30 million, but the wastewater produced there receives virtually no treatment. It is simply used to irrigate land to produce food. Increase in population means increases in the demand for food and for fertilizers to grow this food. The Green Revolution spanning the period from 1967–68 to 1977–78 changed India from a starving nation to one of the world's leading agricultural producers. This change came as a result of harvesting two crops a year from the same land, developing high-yield grains and using more water, more fertilizer, more pesticides, fungicides, and certain other chemicals. The US and Canada produce more than 60% of the surplus food in the world, to make up for the shortfall in other countries. Crop yields in the US have been increased by intensified agriculture, a massive increase in fertilizer application, and installation of under-drainage to leach out the build-up of minerals in the soil. The leachate containing nitrates and phosphorus was discharged to streams and eventually began to enrich the receiving waters. McCarty (1969) described methods used for reducing the discharges of nitrates to San Francisco Bay. The intensified production of meat led to the use of large feed-lots for raising cattle, which resulted in massive discharges of nutrients, mainly nitrogen and phosphorus, to receiving streams.

Discharges of wastewater effluent containing excess nitrogen and phosphorus can contribute to the growth of algae in receiving water, which results in eutrophication.

What is eutrophication?

The term "eutrophication" comes from the Greek word "eutrophos", meaning well-nourished. It referred to the natural ageing of water bodies through the natural addition of nutrients. However in modern terms it means the enrichment of receiving waters with excess nutrients. It is not unusual to find lakes and rivers which have become rich in the main nutrients such as carbon, silicon, nitrogen, and phosphorus, as a result of erosion or runoff from adjacent soils. Other nutrient sources include drainage and wash down of excess nutrients from applied fertilizers, from agricultural feed lots, and domestic and industrial wastes. In the receiving water, these nutrients support for the growth of phytoplankton (algae) which are the first link in the food chain and, hence, the basis of all aquatic life. In surface waters, particularly in oceans, this "primary production" speeds the diffusion of carbon dioxide from the atmosphere to the oceans, the largest sink for carbon dioxide.

The food chain itself consists of many links; each with complex interactions but, in the simplest terms can be described as follows: phytoplanktons are consumed by zooplankton such as daphnia (water fleas) – the food for many species of small fish. These fish are consumed by larger predatory fish which, together with their prey, are food for birds and mammals and, indeed, man. A healthy and well-nourished water body (river, lake, or sea) sustains a rich and diverse aquatic life with all components of the food chain existing in a dynamic equilibrium of production and consumption. A healthy food chain can often survive large changes in nutrient load or climatic conditions with remarkable resilience without any long-term changes in water quality or species diversity.

However, the pressures of expanding population, urbanization, industrialization, and agricultural intensification in many regions have resulted in a massive increase in the loadings of not just nutrients, but also of untreated or secondary treated sewage into the rivers, lakes, and estuaries. Industrial discharges, pesticides, animal wastes, and countless other pollutants can have a direct and devastating effect on the functioning of the food chain.

The combination of greatly increased nutrient input and a wide range of other, potentially ecotoxic, inorganic and organic products that reach the water can have serious effects on the aquatic ecosystem. While primary production of algae is promoted by the increased nutrient supply, the ability of the zooplankton (usually the most pollution-sensitive organisms in the food chain) to respond to this increased food supply is impaired by the presence of other kinds of pollutants. The result is often that the balance of production and consumption in the food chain is disturbed which, in most cases, leads to algae becoming the dominant form of life in the water.

In the worst case, algae will proliferate in a way that can no longer be controlled at the higher levels in the food chain. This may lead to the decline in the populations of other water plants, particularly the bottom-growing plants which fail to obtain adequate light in the turbid-water column. In the most extreme cases, toxic algal scum may be formed and water may become deoxygenated, which will result in fish kills. There are many lakes and reservoirs where elevated nutrient levels have not caused the water quality problems associated with high algal biomass, while other lakes, with similar nutrient loads exhibit signs of algal domination.

An example of such imbalance can be found in the deteriorating condition of Lake Erie in the early 70's which was of particular concern (Knud-Hanson, 1994). The approximately 20,000 pounds of phosphorus per day being discharge into the lake resulted in an about 2,600 square-mile area of the lake with no oxygen within ten feet from the bottom (Beeton, 1971). As of 1967, mats of attached algae covered Lake Erie's shoreline, and the populations of desirable fish such as whitefish, blue pike, and walleye had either severely declined or disappeared altogether.

This was a great concern at the time and even led to a poem:

You're glumping the pond where the Humming-Fish hummed No more can they hum for their gills are all gummed. So I'm sending them off. Oh, their future is dreary They'll walk on their fins and get woefully weary In search of some water that isn't so smeary I hear things are just as bad up in Lake Erie – The Lorax, by Dr. Seuss

The so-called Gulf Anoxia, is caused by excessive amounts of nitrogen being discharged into the gulf from the Mississippi River, which enhances the growth of algae in the gulf. When the algae die, oxygen is consumed, leading to development of a zone where the dissolved oxygen is too low to support fish and other aquatic life, while causing large swings in the pH value as CO2 is extracted or returned to the water. Midsummer coastal hypoxia in the northern Gulf of Mexico was first recorded in the 1970s. From 1993 to 1999 the extent of the bottom-water hypoxia covered between 16,000 and 20,000 km2 and was twice the size of the Chesapeake Bay, rivaling the extensive hypoxia of the Baltic and the Black Sea (Rabelais et al., 2001). This area exceeds the area of the states of Connecticut and Rhode Island.

Much of the initial work on nutrient removal was sparked by the situation around Johannesburg, now known as the Province of Gauteng, South Africa, with close to 10 million people. Because of the discovery of gold, the city is on the continental divide and water is pumped to the city from long distances. In spite of rigid effluent standards, by the early 1970s eutrophication of the reservoirs to the north and south of the city became severe enough to resemble pea soup. Inevitably, as the urban areas grew, recycled wastewater effluent began to constitute an ever higher percentage of the flow to these reservoirs, which in turn supplied water to downstream users. With the salinity of the drinking water reaching a concentration of 800 mg/L during years of drought, the addition of chemicals for the removal of phosphorus was not considered an option. Activated carbon was used to remove tastes and odors at potable water treatment plants.

The concept of a limiting nutrient

Algae, the lowest link in the food chain, need a number of conditions to sustain their growth: sunlight for photosynthesis; an elevated temperature, certain water conditions (turbulence), and nutrients, in particular carbon, nitrogen, and phosphorus which, broadly speaking, are required in the ratio 100:10:1 respectively, as well as a wide range of trace elements. This "primary production" is the foundation of the food chain that sustains all higher life forms: invertebrates, fish, birds, and mammals. If any one of these essential conditions is removed, the primary production ceases and with it, all higher life forms.

In a healthy ecosystem, the ability of the food chain to adapt to variations in nutrient load can be quite remarkable. In its simplest sense, the water body is capable of sustaining a richer and more productive food chain. This can also be achieved without any deterioration in water quality. While the availability of nutrients causes the production of algae to increase, this is balanced by the increase in the consumption of algae by the organisms higher in the food chain that prosper on the increased food supply. Such a situation can often be beneficial by supporting a productive sport or commercial fishery and wildlife.

Since carbon dioxide is freely available from the atmosphere, reducing either nitrogen of phosphorus in discharges to the receiving water to below the values that correspond with the ratio required for growth will limit the growth of the algae. In inland water, phosphorus is mostly the limiting nutrient while in bays and estuaries, nitrogen is predominantly the limiting nutrient, and in some environments both nutrients may be limiting.

Where do the main nutrients come from?

Secondary treatment of wastewater is widely practiced in the USA and Europe, Japan, Australia, and South Africa. Wastewater is treated by exposing it under aerobic conditions to organisms that promote the breakdown of carbonaceous compounds – protein, sugars, soaps, etc. – to carbon dioxide and water. Typically the nitrogen and phosphorus compounds in domestic wastewater are in excess of that required for growth of the organisms in the treatment plant. The excess nutrients are not removed but merely converted from the organic to the inorganic form. The process is depicted on Figure 1. Protein is a combination of many elements but mainly carbon, hydrogen, nitrogen phosphorus, and sulfur. The bound ammonia is converted to ammonia or, when it is further oxidized, to nitrites and nitrate. The phosphorus radical, PO4-3 is discharged with the effluent.

Until 1974, phosphorus used in detergents made up about 50% of the phosphorus in wastewater effluent. After a heated battle between environmentalists and the detergent industry, a number of states outlawed the use of phosphorus in detergents, which resulted in the reduction the phosphorus in treated wastewater effluent from around 11 mg/L to between 5 and 7 mg/L. Phosphorus compounds are being added to some water supplies in older communities where lead pipes are still in use in the water supply system to prevent leaching of lead into the water. In a recent article in Environmental Science and Technology (October 1, 2001) a concern was raised that phosphorus compounds may become too expensive and even be limiting for use for this purpose and the community is urged to recycle and re-use phosphorus. The City of Winnipeg adds 1 mg/L P to the water supply for lead control.

The sources of nutrients that are discharged to water bodies vary from one place to another. Figure 2 shows the nutrient sources in the European Union. Even there, the contribution from human sources equals the contribution from livestock. Those two sources, combined with industry, can be considered "point sources", meaning that nutrients are discharged at discrete points as opposed to dispersed runoff from crop fields, lawns, and parks where fertilizers are applied.

Nitrogen is more often the limiting nutrient for the growth of algae in bays and estuaries and is the main cause of the anoxic conditions in the Gulf of Mexico. Sources contributing nitrogen to the Gulf anoxia are indicated on Figure 3.

Only 11% of the total nitrogen in the Mississippi comes from municipal and industrial point sources in the catchment of the river system which include cities such as Chicago, St Louis, Minneapolis, Kansas City, Nashville, Memphis, and New Orleans, while 15% originates in animal manure and more than 50% comes from fertilizers and mineralized soil through tile drainage systems from crop fields. (Hey et al., 2005) With the expected increase in crop production for the bio-fuel program, the problem will increase in severity. In the Long Island Sound in which the algae growth is also nitrogen-limited, more than 50% of the nitrogen originates in wastewater treatment plants or City of New York, Westchester County and the state of Connecticut. Many US cities have begun removing nitrogen from municipal wastes, but little effort has gone into tackling the bigger problem of removing the nitrogen from agricultural and other diffused sources.

The Las Vegas metropolitan area discharges all wastewater effluent to the Las Vegas Wash and into the arm of Lake Mead that also serves as the area's water supply. Because of the low rainfall in the area, most of the flow into that arm consists of municipal effluent, and the contribution of point sources can be more than 90% of the total. Thus all nutrients contained in the effluent will make up a very high percentage of the total going into that arm of the lake. At present there is sufficient exchange of flow between the Las Vegas Wash and the main body of Lake Mead but as lake levels are dropping, the exchange will be reduced and the effluent of the treatment plants will make up a larger proportion of the domestic water supply.

How do we deal with this surplus of nutrients?

Multiple strategies will be required to reduce the nutrients that affect water bodies. The first line of attack is the removal of nutrients from point sources such as domestic and industrial wastewater treatment plants.

Phosphorus removal: At domestic or industrial waste treatment plants, phosphorus can be removed either by precipitation with metal salts such as aluminum and iron salts which form an insoluble precipitate of aluminum or ferric phosphate, or by incorporation into biological cells. The solids containing the phosphorus can be removed by settling and disposed of with the treated excess biosolids.

In response to public concern about the eutrophication of Lake Erie, pollution control laws were adopted in both countries to deal with water quality problems, including phosphorus loadings to the lakes. In 1972, Canada and the United States signed the Great Lakes Water Quality Agreement to begin a bi-national Great Lakes cleanup that emphasized the reduction of phosphorus entering the lakes. Iron or aluminum salts were added in the wastewater treatment plants to precipitate phosphorus.