Back to Dr. N. V. Hue

Abstract

Production, treatment, and properties of biosolids are discussed, with attention given to Hawaii's products from waster-water treatment plants at Sand Island, Honouliuli, and Kaneohe (Mokapu). Implications of the EPA's 503 rule and impacts of biosolids application to land are examined in terms of soil fertility, crop uptake, water quality, and human health.

Production of Biosolids

Let me begin by asking you a simple question: how much water, on average, do you think you use in a day? 50 gallons, one hundred gallons, or 200 gallons? How many times a day do you flush the toilet? take a shower and clean dishes? The correct answer is about 100 gallons/day. In fact, recent statistics show that Oahu with about 0.8 million people currently generates 133 million gallons of waste water a day, 60% of that or 80 mill. gal. come from residential use.

And as you all know, the Clean Water Act requires that any waste water must be treated to protect human health and the environment before being discharged or reused. As a result, the quantity of waste water residues, now called biosolids but commonly known as sewage sludge, has increased many fold. For example, in 1983 the U.S. generated 6.2 million dry metric tons of biosolids, corresponding roughly to 26 kg per person per year. Biosolids production increased to 9 million ton/year in 1995 and is expected to reach at least 11 million ton/year by the year 2000 as the population grows and more effective wastewater treatment processes are implemented.

In Hawaii, we produce approximately 40 thousand dry tons of biosolids a year. What can we do with them?

Well before we address that question, let's take a quick look at how biosolids are treated and handled, and how safe these materials are.

Treatment of Biosolids

Once pumped into a treatment plant, waste water is first held in a large clarifying tank to remove readily settleable solids. This treatment process produces primary biosolids which contain 3 to 7% solids and can be easily thickened and de-watered. Each thousand gallons of wastewater produce about 3 gallons of primary biosolids. The clarified wastewater may further undergo secondary treatment, which often involves biological processes such as seeding or re-circling biosolids into the wastewater stream as shown here.

In the holding tank,

{CH₂0} + 0₂ ==> CO₂ (gas) + H₂O + microbial biomass

organic N ==> NH₄ + NO₃^-

organic P ==> H₂PO₄^- + HPO₄^2-

This secondary treatment produces secondary or activated biosolids containing 0.5 - 2.0% solids, which are harder to thicken and de-water than the primary biosolids.

Both primary and secondary biosolids are odorous and pathogen rich. They must be stabilized to reduce odor potential and pathogen levels.

Major stabilization processes include anaerobic digestion, aerobic digestion, lime addition and composting. (1) Anaerobic digestion is the most common process, by which biosolids are retained in the absence of air for at least 60 days at 20 C or 15 days at 35 - 55C. This process can bio-degrade 40 - 50% of the volatile solids in the sludge by converting org. carbon to CH₄ and org. N to aqueous NH₃. (2) Aerobic digestion is conducted by agitating biosolids with air or oxygen to maintain aerobic conditions for 60 days at 15C, 40 days at 20C or 10 days at 55 - 60C. Volatile solids are also reduced significantly by this process, but not as much as in the anaerobic digestion. (3) Lime stabilization involves the addition of sufficient CaO or Ca(OH)₂ to biosolids such that pH 12 must be reached after 2 hours of contact. This process would precipitate most heavy metals as well as Ca and phosphate. The high pH also kills most pathogens. (4) Biosolids composting is a process in which biosolids are mixed with a bulking agent, such as wood chips, saw dust or yard waste and allowed to decompose aerobically for several weeks. If operated properly, the temperature in the composting mixture can rise to 55 - 60c and remains hot for several days, effectively killing most pathogens, weed seeds, and parasites.

After stabilization, biosolids are usually dewatered to save transportation and storage costs. Sludge volume can be reduced substantially by partially eliminating its water. For example, raising the solids content from 5 to 10% cuts the sludge volume by half. On the other hand, weight reduction is the primary interest when the solids content exceeds 30%.

Dewatering can be accomplished through centrifugation, vacuum filter, filter press, sand bed drying, or heat drying. Dewatering produces a cake with 15 to 25% solids by centrifugation or vacuum filter, 35 to 50% solids by filter press, 50 to 80% solids by air drying, and 90 to 99% solids by heat drying.

In Hawaii, the two largest wastewater treatment plants at Sand Island and Honouliuli produce primary biosolids with about 25% solids after centrifugation, the Kaneohe (Mokapu) plant produces anaerobically digested biosolids with about 20% solids. Two smaller plants in Waimanalo and Waianae produce secondary digested biosolids with 60 to 80% solids after sand bed drying (Table 1).

Properties of Biosolids.

The next question would come to our mind is that what kind of biosolids do we get? Are they safe to use or even to dispose of? To partially answer that question, let's look at the composition of some biosolids. Pathogens, and to a lesser extent toxic organic chemicals, are important factors in dealing with biosolids. Unfortunately, I have no expertise in those areas. So I will limit the rest of my talk to plant nutrients and heavy metals in biosolids.

Plant Nutrients

As Table 2 shows, plant nutrients such as N, P, K, Ca, Mg in biosolids vary widely. My analysis of Oahu's biosolids in 1991-1992 shows that total N ranges from 1.3 to 6.3%, P from 0.3 to 0.9%, K from 0.01 to 0.1% and so on.

Table 2. Nutrient contents in biosolids

Generally, digested or secondary biosolids contain higher nutrient content than raw or primary biosolids because much of the volatile organic matter has been given off as CH₄ or CO₂ during the digestion process. For example, our Hawaii work shows that the Sand Island and Honouliuli biosolids, which are primary, contain only 1.60% and 2.0 %N whereas the Kaneohe, Waimanalo, and Waianae biosolids, which are anaerobically digested, contain 5.24%N (Table 3).

By averaging all available data together, a so called "typical" biosolids would have 3.2% N, 1.4% P, or 3.1 % P₂O₅, 0.23% K, 2.7% Ca and 0.4% Mg. This information presents several interesting points. (i) First, with respect to N content, biosolids are comparable to chicken manure and could be used as a low grade, slow release N fertilizer, (ii) second, proportion of P to N in biosolids is about one half to one fourth, which is much higher than the P/N ratio in plants. In plants, the P to N ratio is about one fifth to one tenth. Thus, if we use biosolids to meet plant's N requirement, often time we would exceed plant's P needs. Over time, P may build up to a point that P pollution is a strong possibility. (iii) Finally, potassium content of biosolids is inherently low because most K compounds are water soluble and remain in the sewage effluent or the aqueous fraction during biosolids dewatering. Thus, supplemental K fertilizers are likely needed for K poor soils, if the biosolids are to be used as an organic fertilizer.

Heavy Metals.

In my opinion, heavy metal content is the most important factor determining the use of biosolids. And I think the EPA would agree with that view, as the 503 regulation shows. In this rule, 10 metals are regulated. They are arsenic (As), cadmium (Cd), chromium (Cr), Copper (Cu), lead (Pb), mercury (Hg), Molybdenum (Mo), nickel (Ni), selenium (Se), and zinc (Zn).

The middle column lists the maximum concentrations of the ten metals that biosolids must not exceed if land application is the intended use. Biosolids whose metal concentrations fall in between these 2 limits can be land-applied but with some restrictions. In contrast, biosolids whose metal concentrations are below those listed in the right most column can be land-applied as a soil amendment or fertilizer with minimal restrictions; and these restrictions concern only with pathogens. Land applied biosolids must meet either class A or class B pathogen requirements. Class A biosolids have nearly undetectable bacteria counts, meaning that the density of fecal coliform must be less than 1000 most probable numbers (MPN) per gram of dry biosolids or that the density of salmonella sp. must be less than 3 in 4 grams of dry biosolids. For class B biosolids, the density of fecal coliform must be less than 2 x 10⁶ MPN/gram biosolids. To protect public health, the EPA has put several site restrictions on class B biosolids, when they are applied to land. For example, animals must be kept from grazing for at least 30 days, and food crops must not be harvested for at least 14 months (and in some cases as long as 38 months) after biosolids application.

How does the EPA's 503 rule affect Hawaii's biosolids? Frankly not much, at least in the short term. For one thing, our state has not seriously considered using biosolids on land, perhaps because of negative public perception about bio solids. For another, Hawaii's biosolids are fairly low in heavy metals, they would likely meet the exceptional quality (EQ) or clean biosolids category set by the EPA.

As shown in Table 5, our biosolids contain about one half to one tenth the pollutant limits for the clean biosolids category. However, don't forget that most US biosolids are similarly clean. The median concentrations of heavy metals in U.S. biosolids are 8 ppm for Cd, 410 ppm for Cu, 260 ppm for Mn, 45 ppm for Ni and 700 ppm for Zn.

(1) All inorganic N is readily available for crop uptake.

(2) Organic N must be transformed into inorganic N before plants can take up that N. The rate of this transformation which we call N mineralization depends on many factors: sludge quality, soil properties, temperature, and moisture. In warm and humid regions like Hawaii, it is reasonable to assume that:

For primary and aerobically digested biosolids: 40% of organic N would become plant available during the first year after application.

anaerobically digested or chemically stabilized biosolids: 20%

composted biosolids: 10%

biosolid: 3.0% total N & 0.5% (NH₄ + NO₃) -N Target: 200 Kg N/ha

(anaerobic) 2.5% organic N

200/(0.005 + 0.2 x 0.025) = 20,000 Kg /ha

20 ton/ha

So our answer would be about 20 ton/ha. Generally, annual application rates of biosolids to agricultural land range from 5 to 70 ton/ha, with 15 ton/ha being typical.

If your soil is poor in plant nutrients as those soils in central Oahu, then an application of biosolids would make plant grow much better as shown here for a red soil in Wahiawa. Choi Sum grew much better in the plot receiving 45 ton/ha biosolids than the control (Slide). In fact, the beneficial effect is still noticeable more than 10 years after we applied the biosolids. Indeed, we have identified that interactions between organic matter in the biosolids and soil minerals are the cause of these benefits. For example, plants can use P fertilizers recently applied to biosolids-amended plots more effectively than those applied to the control.

However, you need to know a bit about your soil, biosolids, as well as the crop you're going to grow. If you use a biosolid that's low in N and high in C/N ratio such as the Sand Island biosolid and the crop is a fast growing one like tomato, then the presence of biosolids may even hurt your crop in the short term because of N immobilization as shown here. The left pot is the control with no fertilizer added; the center received 2.5% Sand Island biosolid by weight, which is equivalent to about 45 ton/ha, and the right pot received 2.5% Kaneohe biosolid. You can see the difference in growth.

On the other extreme, you may ask what would happen if so much biosolids are applied that more N is released than crops can use? Especially with repeated biosolid applications year after year. Of course, through biological transformation, most of organic N in the biosolids would end up as NO₃^-. NO₃ leaching and ground water pollution is a justifiable concern. Most of our soils carry negative charge, the same as NO₃^-. The two tend to repel each other, meaning that NO₃ movement in soil is fairly fast. Hawaii soils with high clay content and many micropores, retart NO₃ movement somewhat. Nevertheless, NO₃^- does move downward.

Why are we so concerned about nitrate pollution of ground water? In my opinion, that concern comes partially from the fear that if nitrate can move to ground water then something else like pesticides can too. In reality, nitrate itself is not toxic to living organisms but nitrite (NO₂^-) is. In babies of 6 months old or younger, their stomach has certain bacteria that can convert NO₃^- to NO₂^- . Nitrite can then oxidize Fe²⁺ in hemoglobin to Fe³⁺ , converting hemoglobin to methemoglobin which is a poor O₂ carrier. As the O₂ carried by the blood decreases, the body is suffocated and turns blue. Thus, the name "blue-baby" syndrome came about. While most infants can tolerate NO₃^- in water at levels much higher than 10 mg N/L, which is the EPA's standard for drinking water, some infants begin to exhibit methemoglobin symptoms at levels only slightly higher than 10 mg N/L after a few days of consumption. Adults and older children are much less susceptible to MeHb problem because stomach HCl levels increase with age, and HCl kills most of the bacteria that can reduce NO₃^- to NO₂^-. For adults, 10 mg NO₃^-N/L is a lifetime health advisory level. The rationale for this advice is that, although NO₃^- itself is not carcinogenic, a small fraction of it could be reduced to NO₂^- and react with secondary and tertiary amines in the human body to form nitrosamines. Nitrosamines have been identified as a potent carcinogen in animals and may be an important carcinogen in humans. However, there is no conclusive evidence linking any type of cancer to NO₃^- in drinking water.

Heavy Metals

What about heavy metals? Unlike nitrogen and P, which is an asset, heavy metals are a liability of biosolids. It is interesting to note that before 1993 when the EPA's 503 rule came into effect, only Cd was regulated in biosolid use. Now 10 metals in biosolids are regulated. Because of time constraints, we're going to focus only on Cd and Mo, in this discussion.

Two important factors that we should be aware of when working with heavy metals in biosolids and in soils.

1. Pound for pound, the bio-availability of metals in biosolids is much lower than that of a pure metal salt, say sludge-contained Cd vs CdCl₂ or CdSO₄. (Slide). The reason is that most metals in biosolids are in solid forms with very low solubility. They are either precipitated by carbonate and/or sulfide, complexed by organic matter, or sorbed by Fe, and Mn oxides. The ceiling concentration limits for the 10 heavy metals in the 503 rule, that we discussed earlier, were set as such partially because the EPA worried that when heavy metals in biosolids are high enough, they may act like a metal salt, thus cause severe toxicity to plants and animals.

2. Except for Mo, most other heavy metals are more water soluble and thus more bio-available when soil pH decreases (that is when the soil becomes more acidic). As shown here for Zn (slide), at pH 7, the limit of 1 mg Zn/L in soil solution, a level that may cause slight reduction in growth of some vegetables, would be attained at about 1200 ppm Zn in soil. At pH 6, that soluble Zn level would be reached with 100 ppm Zn in soil. And at pH 5, it takes only 40 ppm Zn in soil to maintain 1 ppm Zn in solution. That's why it is recommended that soil pH be adjusted to between 6.5 and 7.0 when biosolids are applied and be maintained at 6.0 or above as long as the land is used for production of crops for human or animal consumption.

We're concerned about heavy metals because at elevated levels they are harmful to humans and animals. Let's use Cd as an example since this metal has been studied most. The potential risk of high Cd in food to human health was first recognized in the early 70's when Cd-related "itai-itai" disease was reported. In the 60's a large number of Japanese farmers suffered Cd health effects after long-term consumption of Cd enriched rice grown in paddies irrigated with Cd polluted water (from the Jinzu river). Among those farmers, there was a strong correlation between kidney problems, such as high levels of proteins and sugars in urine, and Cd concentration in rice they ate. By the way, the name "itai-itai", meaning ouch-ouch in English, came from expressions of pain by women suffering Cd-induced bone fractures.

Cd accumulates most in kidneys, and to a lesser extent in liver, with a mean residence time of about 30 years. Thus, the early health effect of excessive chronic Cd exposure is the renal dysfunction or kidney disease. It is believed that much of the physiological effect of Cd arises from its chemical similarity to Zn. Cd may replace Zn in some enzymes, thereby altering the stereostructure of the enzyme and impairing its catalytic activity.

Molybdenum is another interesting story. At low concentrations (say, less than 5 ppm in plant biomass), Mo is an essential nutrient for plants and animals. Yet, at high concentrations in forage, Mo can poison grazing animals. Ingested Mo is transformed in the rumen to tetrathiomolybdate, which binds Cu strongly, preventing Cu absorption by the gut and reducing Cu bio-availability. If forages contain 5 to 10 ppm Mo such that Cu/Mo ratio is less than 2, induced Cu deficiency may occur. Generally, the availability of Mo for plant uptake increases and that of Cu decreases as soil pH rises. Thus, Mo toxicity to animals is most frequently associated with forage grown on alkaline soils.

Concluding Remarks

We have discussed biosolids treatment and properties as well as their impacts -- both good and bad -- on man and the environment. In conclusion, the following points should be emphasized.

On the positive side: (1) biosolids N, which is mostly organic, is less likely to cause ground water pollution than chemical N fertilizers. (2) Other nutrients, particularly P, are also valuable. In some soils, such as Fe-deficient calcareous soils, biosolids can provide many essential micro nutrients (e.g. Fe, Zn, Cu) even more effectively than commercial fertilizers. (3) The organic matter in biosolids can adsorb and deactivate heavy metals to such an extent that an exceptional quality class of biosolids has been legally established.

On the negative side: biosolids contain a "soup" of metals, organic chemicals and pathogens. Most of these pollutants have no known adverse effects, but some show inconclusive, thus worrisome effects, while a few have been proven hazardous to human health or the environment. Also, economically the build-up of heavy metals from land-applied biosolids may devaluate the land in the future. Obviously, risks are involved in using biosolids on land. A delicate balance must be sought between the needs for re-use or disposal of biosolids and the responsibility to protect public health and the environment.

But, if other states can do it, so can Hawaii. Right?

Back to Dr. N. V. Hue