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...