Adjusting N:P ratios in liquid dairy manure through nitrification and chemical phosphorus removal to match crop fertilizer requirements

BackgroundNutrient recycling through land-application of manure is an economical and practical farming practice. A nitrogen to phosphorus (N:P) ratio of 5:1, which is suitable for crop application, is typical in fresh manure. If left untreated, nitrogen (N) in the manure is subject to loss via volatilization, denitrification, and runoff. Conservation of nitrogen in a non-volatile form will help maintain the fertilizer value of manure.

Phosphorus-based nutrient management plans help prevent phosphorus (P) loss via runoff which can occur as P accumulates in soil over time. As the amount of cropland available for manure application decreases, it is important that manure be treated for phosphorus removal so that more liquid manure can be utilized and nitrogen needs can be met without the risk of over applying phosphorus.

Objectives• Develop a cost effective treatment strategy to

conserve nitrogen in liquid dairy manure by determining (1) the most cost-effective aeration strategy to concentrate nitrogen in the manure and (2) the effects of recycled flush water on bio-available N during nitrification.

• Evaluate the use of chemicals to reduce phosphorus concentrations in the treated dairy manure to achieve suitable N:P ratios for crop production.

StrategyNitrification is the oxidation of ammonia to nitrate by aerobic autotrophic bacteria. This process is used to conserve nitrogen in a non-volatile form.

NH4+ + 2O2 NO3

- + 2H+ + H2O

The goal is to maximize nitrogen conservation while minimizing nitrogen loss and energy usage through the use of various intermittent aeration strategies.

MethodsThree 30 L attached growth reactors were constructed (Figure 1). The reactors are filled with 16 mm diameter Norpac® media (Figure 2) and each contains two diffuser stones providing air at approximately 2 L/min.

Figure 2. Media for attached growth of biomass

Figure 1. Storage tank and nitrification reactors

ResultsThe 100%, 75%, and 50% aeration strategies have been tested to date. Table 2 shows the average characteristics of both the influent and effluents. Although the effluent total nitrogen (TN) was lowest at the 50% aeration treatment, the low nitrate concentration indicates nitrification was not the reason for nitrogen reduction. Ammonia stripping was the most likely cause of such significant nitrogen loss.

Work to be done• The aeration strategies previously described will be

repeated using scraped/separated dairy manure. The goal is to determine if flush water affects the bio-availability of nitrogen during nitrification.

• N:P ratios will be adjusted by phosphorus removal. Reactor effluent will undergo batch tests for phosphorus removal by chemical precipitation using aluminum- and iron-based salts and polymers. Efficiency of phosphorus recovery and sludge production will be quantified.

Once steady state was reached, the 100% and 75% reactors performed similarly in terms of nitrate production. The 75% intermittent aeration strategy appears to be a viable and economical option for liquid manure treatment (Figure 3).

Figure 5. Reactor maintenance

J. DeBusk1, J. Arogo1, N. Love2, and K.F. Knowlton3

1 Biological Systems Engineering2 Civil and Environmental Engineering3 Dairy Science

Flushed and separated liquid dairy manure is obtained from the Virginia Tech dairy complex. The manure is stored in a 120 L anaerobic stirred tank prior to being fed to the reactors. The reactors are maintained at a hydraulic retention time of approximately 3.75 d. Analysis is done on reactor influent and effluents; effluents are sampled three times per week. The reactors are aerated according to the aeration schemes shown in Table 1.

Figure 3. Effluent nitrate from reactors receiving 100%, 75%, and 50% aeration treatments

Figure 4. Distribution of nitrogen in raw and treated liquid manure

A significant reduction in total ammonia nitrogen (TAN) was observed in all three reactors, but only the higher aeration strategies achieved the goal of nitrogen conservation as nitrate (Figure 4). Further analysis is needed to determine the means of total nitrogen losses.

 

Table 2. Average characteristics of influent manure and treated effluent from reactors

8.438.238.368.23pH

58120NO2-N, mg/L

171871190NO3-N, mg/L

1012064572NH3-N, mg/L

369478487933TKN, mg/L

391673619933TN, mg/L

164253214295TP, mg/L

2971435540946236VSS, mg/L

4037600956478755TSS, mg/L

4629639762639020VS , mg/L

8847109721116414516TS, mg/L

2747246326904612Alkalinity, mg/L

73418765844112794COD, mg/L

50% Effluent

75% Effluent

100% EffluentInfluent

8.438.238.368.23pH

58120NO2-N, mg/L

171871190NO3-N, mg/L

1012064572NH3-N, mg/L

369478487933TKN, mg/L

391673619933TN, mg/L

164253214295TP, mg/L

2971435540946236VSS, mg/L

4037600956478755TSS, mg/L

4629639762639020VS , mg/L

8847109721116414516TS, mg/L

2747246326904612Alkalinity, mg/L

73418765844112794COD, mg/L

50% Effluent

75% Effluent

100% EffluentInfluent

Table 1. Aeration strategies for the nitrification reactors

60 min60 min50%

40 min60 min60%

20 min60 min75%

-continuous100%

Time OFFTime ON% Aeration

60 min60 min50%

40 min60 min60%

20 min60 min75%

-continuous100%

Time OFFTime ON% Aeration